123 
A7 


UC-NRLF 


B  M  170  420 


GIFT  OF 


SCIENCES 
LIBRARY 


REPORT 


ON 


LJ  I  13  li  A 

; 
CALIFORNIA 


THE  GEOLOGICAL   MAP 


OF 


MASSACHUSETTS, 


PREPARED     BY 


W.   0.   CROSBY, 


ASSISTANT    IN    THE    LABORATORY    OF    THE    BOSTON    SOCIETY   OF 
NATURAL    HISTORY. 

PROF.    ALPHEUS  HYATT  IN   CHARGE. 


UNDER  THE  DIRECTION   OF    THE    MASSACHUSETTS  COMMISSION 
TO  THE   CENTENNIAl  EXPOSITION. 


BOSTON: 

PRESS  OF  A.  A.  KINGMAN. 
1876. 


REPORT 


ON 


THE  GEOLOGICAL  MAP 


OP 


MASSACHUSETTS, 


PBEPABED    BY 


W.   0.   CROSBY, 


ASSISTANT    IN    THE    LABORATOBY    OP    THE    BOSTON    SOCIETY  OP 
NATUBAL    HISTOBY,  PBOF.    ALPHEUS    HYATT 


UNIVERSITY  OF 

CALIFORNIA. 

UNDER  THE  DIRECTION   OF    THE    MASSACHUSETTS  COMMISSION 
CENTENNIAL  EXPOSITION. 


Distributed  by  the  Boston  Society  of  Natural  History  at   the 
request  of  the  Centennial  Commissioner  for  Massachusetts. 


BOSTON: 

PRESS  OF  A.  A.  KINGMAN. 
1876. 


,<  :-\Vf  K. 


if 
X 


LI  BEAR  Y 

UNIVERSITY  OF 

CALIFORNIA. 


HON.  LEVERETT  SALTONSTALL, 

Massachusetts  Commissioner  to  the  Centennial  Exposition : 

SIR  :  I  have  the  honor  to  present  herewith  the  Report  on 
the  Geological  Map  of  Massachusetts,  prepared  under  the  di- 
rection of  the  Commission. 

My  effort  has  been  to  unite  the  latest  and  most  reliable 
information,  whether  published  or  unpublished,  and,  with 
these  results,  to  give  such  a  view  of  the  Geology  of  the  State 
as  would  represent  the  advances  made  since  the  Wall  Map 
of  Prof.  Edward  Hitchcock  was  published  in  1844. 

Over  and  above  the  function  of  directing  the  work,  and 
securing  the  execution  of  this  plan,  I  do  not  assume  any 
credit  for  the  collection  of  the  materials,  or  the  results  reached 
by  this  means.  Most  of  the  outlines  of  the  formations  remain 
substantially  as  they  appear  on  Prof.  Hitchcock's  map;  though 
many  of  the  details,  and  almost  the  entire  eastern  portion  of 
the  State,  will  be  found  to  have  been  much  changed,  when 
compared  with  any  previous  map.  For  these  and  other  mat- 
ters of  original  and  unpublished  interest,  the  Commission  is 
indebted  to  the  voluntary  labors  of  Mr.  W.  O.  Crosby,  Assist- 
ant in  the  Laboratory  of  the  Boston  Society  of  Natural  His- 
tory, Mr.  L.  S.  Burbank,  of  Woburn,  Mr.  T.  T.  Bouve,  Presi- 
dent of  the  Boston  Society  of  Natural  History,  Prof.  N".  S. 
Shaler,  of  Harvard  University,  Prof.  W.  H.  Niles,  of  the 
Massachusetts  Institute  of  Technology,  Prof.  B.  K.  Emerson, 
of  Amherst  College,  Mr.  W.  W.  Dodge,  of  Cambridge,  Mr. 
F.  W.  Very,  of  Dedham,  Mr.  W.  W.  Jacques  and  Mr.  C.  W. 
Kempton,  both  of  Newburyport,  Mr.  Mathew  Anderson,  of 

332696 


Cambridge,  and  others.  The  particular  facts  which  they 
have  contributed  will  be  found  in  the  body  of  the  text.  In- 
formation with  regard  to  the  eastern  boundary  of  the  Miocene 
on  the  island  of  Martha's  Vineyard,  and  some  other  points, 
were  contributed  by  Prof.  N.  S.  Shaler,  from  maps  prepared 
by  him  for  the  United  States  Coast  Survey,  and  our  acknowl- 
edgments are  also  due  to  them. 

Valuable  and  efficient  aid,  without  which  much  of  the  work 
would  necessarily  have  remained  undone,  has  been  received 
from  the  Presidents  and  Directors  of  the  following  Railroad 
Companies,  in  the  form  of  free  passes  over  their  respective 
lines  :  the  Eastern,  Fitchburg,  Old  Colony,  Boston  and  Provi- 
dence, .and  Connecticut  River.  The  Boston  Society  of  Nat- 
ural History  has  also,  in  the  loss  of  the  services  of  the 
Custodian  and  his  Assistant,  while  engaged  in  the  construc- 
tion of  the  Map,  contributed  largely  to  its  successful  comple- 
tion. 

Mr.  Crosby  has  been  much  in  the  field,  the  weather  having 
been  very  favorable  during  the  past  winter.  Mr.  Burbank 
and  others  have  also  spent  considerable  time  in  this  way  In 
fact,  everything  has  been  done  to  make  the  original  work 
accurate,  which  the  time  and  the  necessarily  limited  appro- 
priation would  permit. 

With  great  respect, 

Your  Ob't  Serv't, 

ALPHEUS  HYATT, 
Custodian  Boston  Soc.  JVat.  Hist. 


EEPOET 

ON   THE 

GEOLOGICAL  MAP  OF  MASSACHUSETTS. 

BY  W.  0.  CROSBY.1 


In  1830  the  Government  of  Massachusetts  instituted,  at  the 
public  expense,  a  Geological  Survey  of  the  Commonwealth, 
and  Prof.  Edward  Hitchcock,  Sen.,  of  Amherst  College,  was 
charged  with  the  work  of  exploration.  His  final  report,  ac- 
companied by  a  Geological  Map  of  the  State,  was  published 
eleven  years  later,  in  1841.  This  map,  enlarged,  and  revised 
by  Prof.  Hitchcock,  was  republished  in  1844,  in  connection 
with  the  Topographical  Wall  Map  of  Simeon  Borden.  No 
Geological  Map  of  Massachusetts  materially  differing  from 
this  has  since  appeared.  The  work  of  Prof.  Edward  Hitch- 
cock, Sen.,  remains  the  last  general  contribution  to  the  geol- 
ogy of  the  State ;  and  the  map  to  which  the  present  report 
relates — which,  since  it  has  been  prepared  under  the  direction 
of  the  Massachusetts  Commission  to  the  Centennial  Exposi- 
tion, may,  for  the  sake  of  a  convenient  designation,  be  called 
the  Centennial  Map — is  based  primarily,  as  all  subsequent 
maps  must  necessarily  be,  upon  those  published  by  him.  It 
should  be  stated  in  this  connection,  however,  that  some  as- 

1  Those  parts  of  this  report  which  relate  to  that  portion  of  the  State  lying  east 
of  Worcester  and  north  of  Rhode  Island  have  been  taken  from  my  thesis  for 
graduation  in  the  Mass.  Institute  of  Technology,  on  the  "  Geology  of  Eastern 
Massachusetts,"  which  will  be  published  during  the  ensuing  summer  in  the  Pro- 
ceedings of  the  Boston  Society  of  Natural  History. 


G 


sistance  has  been  derived  from  the  geological  map  of  Prof. 
C.  H.  Hitchcock,  which  was  published  in  1871  in  Walling's 
Atlas  of  Massachusetts,  and  differs  in  some  points  of  minor 
importance  from  the  earlier  maps  referred  to. 

It  is  proposed  to  embody  in  this  report  a  brief  account  of 
all  the  observations,  by  whomsoever  made,  upon  which  the 
changes  appearing  in  this  Centennial  Map,  as  compared  with 
previous  maps,  are  based;  with  the  exception  of  the  data  fur- 
nished by  Mr.  L.  S.  Burbank,  whose  researches  in  the  Nashua 
and  Merrimac  Valleys  have  been  of  very  material  assistance, 
and  will  be  found  fully  described  in  his  report  which  accom- 
panies this. 

In  some  instances,  as  in  the  eastern  part  of  Plymouth 
county,  the  alterations  of  boundaries  are  based  upon  the  ob- 
servations of  Prof.  Edward  Hitchcock,  published  in  his  final 
report  of  1841,  but  of  which  different  views  have  been  taken. 
In  such,  as  in  all  other  cases,  the  sources  of  the  data  used 
will  be  clearly  indicated. 

My  own  contributions  to  the  map  are  based  upon  observa- 
tions extending  over  a  period  of  four  years,  and  during  the 
last  year  I  have  been  in  the  field  almost  constantly.  It  is 
only  justice  to  Dr.  T.  Sterry  Hunt  to  express  here  a  recog- 
nition of  my  indebtedness  to  him  for  much  valuable  instruc- 
tion and  advice,  while  a  student  in  the  Massachusetts  Insti- 
tute of  Technology,  without  which  the  performance  of  this 
work  by  me  had  been  impossible.  But,  though  placed  in  a 
position  most  favorable  to  a  thorough  acquaintance  with  Dr. 
Hunt's  views  concerning  the  origin  of  crystalline  rocks  and 
their  relations  to  the  more  recent,  fossiliferous  rocks,  I  en- 
tered the  field  a  year  ago  with  a  general  disbelief  in  the 
applicability  of  these  ideas  to  the  rocks  of  this  region,  which 
my  first  observations  only  strengthened ;  and  any  confirma- 
tions of  these  views  which  may  be  found  in  this  report,  have 
been  reached  in  opposition  to  preconceived  ideas  wholly  at 
variance  with  them. 


That  enormous  period  in  the  history  of  the  earth,  commonly 
known  as  geological  time,  is  usually  divided  by  geologists 
into  four  great  eras,  which,  in  their  order  of  sequence,  are 
the  Eozoic,  Paleozoic,  Mesozoic,  and  Cenozoic ;  and  in  Mass- 
achusetts rocks  are  found  belonging  to  each  of  these  grand 
divisions  of  past  time.  The  rocks  of  Massachusetts  are  mainly 
crystallines,  which  are  believed  to  belong  wholly  to  the  Eozoic 
era ;  and  it  is  a  noteworthy  fact,  that  the  rocks  belonging  to 
the  successive  eras  in  this  State  steadily  diminish  in  amount 
from  the  Eozoic  to  the  Cenozoic,  so  that?  Massachusetts  is,  in 
this  respect,  an  epitome  of  the  world.  The  oldest  rocks  in 
Massachusetts,  as  will  be  presently  shown,  are  on  its  eastern 
border,  facing  the  Atlantic;  and  proceeding  westward  across 
the  State  the  crystallines  become  gradually  newer,  until,  in 
Berkshire  county,  we  find  the  semi-crystalline  beds  believed 
by  an  increasing  number  of  geologists  to  represent  the  close 
of  Eozoic  time. 

The  Eozoic  rocks  of  Massachusetts  may,  to  a  large  extent 
at  least,  be  divided  lithologically  and  chronologically,  into 
three  divisions,  which,  stated  in  their  order  of  sequence,  are 
the  Norian,  the  Huronian,  and  the  Mont  Alban.  I  weigh  my 
words  well  when  I  describe  these  divisions  as  both  litholog- 
ical  and  chronological;  for,  although,  as  already  stated,  I 
began  my  study  of  the  crystallines  of  this  State  with  a  whole- 
some distrust  of  the  value  of  lithological  data  in  establishing 
chronological  divisions,  and  am  not  yet  wholly  prepared  to 
apply  this  principle  to  wider  regions,  I  do  not  hesitate  to 
affirm  that  the  lithological  characters  of  the  divisions  which 
have  been  worked  out  among  the  crystallines  of  this  region  — 
the  chronological  and  geographical  distinctness  of  which  I 
cannot  doubt  —  are  as  unlike  as  the  faunae  of  any  two  succes- 
sive geological  formations. 

NOBIAN. 

But  two  small  areas  of  the  rocks  of  this  age  have  thus  far 
been  found  in  Massachusetts.  The  largest  and  most  important 


8 


includes  the  city  of  Salem,  Salem  Neck,  the  islands  of  Great 
Misery  and  Little  Misery,  Baker's  Island,  Naugus  Head  on 
the  north  end  of  Marblehead,  various  small  islands  between 
Marblehead  and  Great  Misery,  and  several  narrow  strips 
along  the  Beverly  shore.  The  second  area  includes  all  but 
the  seaward  end  of  large  Nahant.  The  rocks  of  this  forma- 
tion, though  frequently  stratified,  seem  in  general  to  have 
been  somewhat  fluent,  and  usually  exhibit  more  or  less  ex- 
travasation; but  doubtless  in  some  cases  the  metamorphic 
action  has  stopped  Ishort  of  this  extreme  term,  though  de- 
stroying all  traces  of  bedding.  In  many  places,  as  notably 
on  Winter  Island  and  Great  and  Little  Misery,  the  entire 
formation  seems  to  have  been  fluent,  and  the  extravasation 
has  been  so  extensive  that  the  character  of  the  rock  changes 
nearly  every  rod.  One  important  fact  should  be  noted  here, 
viz. :  nowhere  in  this  region  does  the  Norian  series  appear  to 
be  cut  by  eruptives  belonging  to  another  formation,  for  all 
the  extravasated  rocks  of  this  system  may  be  easily  referred 
to,  or  shown  to  be  derived  from,  its  stratified  members. 

The  stratified  rocks  occur  chiefly  on  Marblehead,  and  on 
the  Beverly  shore,  west  of  Curtis  Point.  On  Marblehead  the 
strike  is  E.-W.,  with  a  vertical  dip,  while  on  the  Beverly  shore 
the  strike  varies  from  N.-S.  to  N".  E.-S.  W.,  and  the  dip  is 
30°  to  vertical  to  the  N.  W.  The  average  strike  of  the  whole 
system  is  N.  E.-S.  W.  More  or  less  distinct  bedding  has  also 
been  observed  on  the  north  side  of  Great  Misery,  at  several 
points  on  Salem  Neck,  on  Coney  Island,  and  at  one  point  on 
the  north-east  shore  of  JSTahant. 

The  rocks  of  this  formation  are  composed  chiefly  of  feld- 
spar and  hornblende,  or  pyroxene.  These  minerals  occur 
mixed  in  very  various  proportions.  It  has  been  proved  by 
analysis  that  some  of  the  feldspar  is  not  labradorite,  though 
closely  resembling  that  species  in  some  of  its  physical  char- 
acters ;  and  it  yet  remains  to  be  proved  that  there  is  any 
labradorite  in  the  formation.  But  its  occurrence  here  is 
deemed  probable,  and  as  only  one  analysis  has  been  made, 


9 

the  question  cannot  be  regarded  as  settled.  It  should  be 
said,  however,  that  all  the  feldspars  of  this  formation  agree 
very  closely  in  their  general  physical  properties  with  the 
specimen  analyzed.  This  was  placed  in  the  hands  of  Mrs. 
Richards,  of  the  Massachusetts  Institute  of  Technology,  who 
made  the  following  report : 

SiG2         .        .        .        ...        .        . 

A19O3   .        .        .       V.   ;   -.  *      -. 

K2b    &i     .      . .    '*      . v  . 

Na2O , 

CaO      -_V  •     *        » •.,.:•     > 

H20  ,  *     £        .      >,,    ... 

100.525 

Such  a  chemical  constitution  belongs  to  no  known  species 
or  variety  of  feldspar ;  and  fearing  there  had  been  an  error 
in  the  first  analysis,  a  second  was  made  from  the  same  piece, 
a  very  clear  unweathered  specimen,  with  a  like  result. 

A  careful  inspection  of  a  large  number  of  specimens  ex- 
plained the  anomaly,  by  showing  that  the  feldspar,  although 
coarsely  and  perfectly  crystalline,  is  a  mixture ;  the  crystals 
being  formed  of  thin  interlaminated  plates  of  at  least  two 
different  feldspars.  This  can  be  seen  only  on  slightly  weath- 
ered surfaces;  one  of  the  feldspars  decomposing  more  read- 
ily than  the  other.  The  constituent  feldspars  were  not 
analyzed,  as  it  seemed  impracticable  to  separate  them.  But 
if,  as  appearances  indicate,  it  is  a  mixture  of  two  species 
only,  they  are  probably  an  orthoclase  rich  in  soda,  and  an 
oligoclase  poor  in  lime,  the  latter  predominating.  The  color 
of  this  feldspar  is  bluish  gray,  weathering  white,  and  its  sp. 
gr.  varies  from  2.55  to  2.60.  Disseminated  through  this  feld- 
spar, are  numerous  small  crystalline  grains  of  magnetite. 
Biotite  is  not  uncommon  in  the  rocks  of  this  formation, 
though  seldom  abundant.  Pyroxene  is  abundant,  and  the 
varieties  hypersthene  and  bronzite  are  believed  to  occur.  A 
massive,  coarsely  crystalline  diorite,  at  Nahant,  is  decidedly 
epidotic.  The  most  striking  and  probably  the  most  impor- 
2 


10 

tant  mineralogical  character  of  this  series  of  rocks  is  that  all 
members  of  it  are  absolutely  destitute  of  quartz. 

Perhaps  the  most  characteristic  rock  of  this  formation  is 
one  composed  almost  entirely  of  feldspar,  and  containing 
little,  frequently  no  hornblendic  material.  It  is  usually  coarse- 
ly crystalline,  this  variety  predominating  along  the  Beverly 
shore,  east  of  Curtis'  Point ;  and  the  crystallization  is  some- 
times extremely  coarse,  as,  notably,  on  Marblehead,  especially 
about  Dolliber's  Point,  where  it  is  interstratified  with  fine 
grained  pyroxenic  rocks;  but  it  is  frequently  fine  grained, 
resembling  felsite,  and  presents  a  very  uniform  texture  and 
appearance.  This  fine  grained  variety  occurs  at  many  points, 
but  is  most  largely  developed  on  Baker's  Island,  which  is 
principally  formed  of  it. 

A  dark,  heavy,  usually  coarse  grained  diorite,  varying 
greatly  in  composition,  and  frequently  occurring  as  a  pure 
hornblende  rock,  is  the  most  abundant  of  our  Norian  rocks. 
It  is  the  prevailing  rock  on  Nahant,  where  it  is  frequently 
very  coarse  and  hornblendic,  with  a  high  specific  gravity.  A 
more  feldspathic  variety  of  this  rock  exhibits  traces  of  bed- 
ding on  the  north-east  shore  of  Nahant,  as  noticed,  ante  p.  8. 
This  diorite,  and  the  feldspathic  rock  just  noticed,  occurring 
chiefly  as  eruptives,  underlie  the  city  of  Salem,  and  form  the 
peninsula  of  Salem  Neck,  and  the  islands  of  Great  Misery, 
Little  Misery,  Pope's  Head,  and  Eagle  Island. 

The  stratified  rocks  of  this  series  are  mainly  fine  grained, 
and  consist  usually  of  interstratified,  frequently  alternating, 
pyroxenic  and  feldspathic  beds.  Among  the  stratified  rocks 
are  found  those  of  all  textures,  from  the  finest  to  the 
coarsest;  the  stratified  pyroxenic  rocks  are,  however,  usually 
fine  grained  and  very  distinctly  bedded. 

That  this  series  of  pyroxenic  and  feldspathic  rocks,  with 
its  associated  minerals,  which  is  sometimes  stratified,  oftener 
eruptive,  frequently  very  coarsely  crystalline,  and  always 
quarteless,  agrees  better,  even  in  the  absence  of  labradorite, 
with  the  Norian  system  than  any  other  that  has  been  de- 


11 

scribed,  can  scarcely  be  doubted.  Other  considerations  con- 
firm this  view.  The  great  disturbance  which  it  shows,  and 
its  thoroughly  crystalline  appearance,  stamp  it  as  older  than 
the  Huronian  and  Mont  Alban,  and  it  also  clearly  underlies 
these  formations,  and  has  been  frequently  extravasated 
through  them.  This  is  well  shown  in  the  cliffs  on  the  shores 
of  Beverly  and  Manchester,  where  the  Huronian  granites  are 
cut  extensively  by  great  dykes  and  eruptive  masses  of  No- 
rian  rocks,  both  hornblendic  and  feldspathic  ;  the  feldspathic 
dykes  appearing,  usually,  to  be  older  and  larger  than  the 
hornblendic.  The  Norian  eruptives  and  Huronian  diorites 
are  similarly  related  on  Marblehead.  At  Nahant  we  find 
slates,  believed  to  be  of  Primordial  age,  resting  upon  the 
Norian  diorites,  which  have  been  extravasated  through  the 
slates,  producing  extensive  alterations.  The  coarse  grained, 
readily  disintegrating,  exotic  diorites,  so  extensively  quarried 
in  Medford,  and  also  occurring  in  Somerville  and  Brookline, 
are,  doubtless,  extruded  portions  of  this  same  series,  which  is 
the  probable  seat  of  many  of  the  eruptive  rocks,  especially 
diorites,  cutting  the  newer  formations.  We  thus  find  this 
series  underlying  everything  newer  than  the  ETorian ;  and, 
since  the  Laurentian  is  not  known  to  occur  in  Massachusetts, 
we  are,  I  believe,  fully-justified  in  regarding  this  series,  not 
only  as  the  Norian,  but  also  as  the  oldest  series  in  the  State. 
The  student  of  Massachusetts  geology,  may  find  here  a 
chronological  and  stratigraphical  starting  point.  Here  is  the 
real  base  of  our  geological  column. 

It  should  be  stated  that  Prof.  Hyatt,  long  ago,  recognized 
the  rocks  about  the  city  of  Salem,  as  probably  older  than  the 
felsites  of  Marblehead  Neck  ;  but  did  not  separate  them  from 
the  Huronian  diorites  of  Salem  Swampscot  and  Marblehead. 
Dr.  T.  Sterry  Hunt,  also,  in  1869,  identified,  as  of  Norian 
age,  a  boulder  found  near  Salem  and  probably  derived  from 
the  Norian  rocks  of  that  vicinity. 

'"LIBRARY 

UNIVERSITY  OF 

CALIFORNIA. 


12 

HURONIAN. 

The  rocks  of  this  age,  like  those  of  the  Norian,  occur  only 
in  the  eastern  portion  of  the  State,  where  they  cover  a  wide 
area,  forming  the  seashore  from  the  New  Hampshire  line  to 
Plymouth.  They  are  bounded  on  the  west  by  a  line  running 
south-westerly  from  Salisbury  through  Essex  and  Middlesex 
counties  to  Concord ;  here,  after  giving  off  a  long  and  narrow 
deflection  which  continues  nearly  twenty  miles  farther  to  the 
south-west,  the  line  bends  to  the  south  and  continues 
through  Framingham,  Holliston,  Medway  and  Bellingham 
to  the  north-east  corner  of  Rhode  Island.  The  Huroniaii 
area  has  an  extreme  length,  measured  from  the  New  Hamp- 
shire line  in  Salisbury,  to  Manomet  Hill  in  Plymouth,  of 
sixty-five  miles,  and  an  extreme  breadth,  measured  across 
the  southern  end,  not  counting  the  narrow  band  stretching 
from  Concord  to  Westborough,  of  about  forty  miles.  It  is 
partially  interrupted,  near  the  middle,  by  an  area  of  Prim- 
ordial, and  more  recent  rocks,  which  lie  about  the  shores  of 
Boston  Harbor,  extending  westerly  to  Natick,  and  south- 
westerly to  Rhode  Island.  The  Huronian  on  this  map 
comprises  the  areas  marked  as  "syenite,"  "porphyry,"  and 
"  hornblende  slate,"  on  the  geological  map  of  Prof.  Edward 
Hitchcock.  In  the  State  Cabinet,  collected  by  Prof. 
Edward  Hitchcock,  are  numerous  specimens  of  hornblendic 
granite  ("  syenite " )  from  Kingston,  and  Manomet  Hill  in 
Plymouth,  and  from  the  latter  locality,  also,  several  speci- 
mens of  felsite  ("porphyry").  These  are  typical  Huronian 
rocks  and  I  have,  in  consequence  of  their  occurrence  at  the 
localities  named,  ventured  to  extend  the  Huronian  formation, 
on  the  map,  as  far  south  as  a  line  running  westerly  from  the 
southern  end  of  Manomet  Hill ;  although  on  all  previous 
maps  the  towns  of  Plymouth  and  Kingston  are  represented 
as  entirely  covered  with  "granite"  (micaceous  granite,  of 
Mont  Alban  age). 


13 


Prof.  C.  H.  Hitchcock,  in  his  recent  report  (1875)  on  the 
Geology  of  New  Hampshire,  has  applied  the  term  Labrador 
to  the  rocks  of  this  broad  Huronian  area ;  although  these 
rocks  iiave  scarcely  a  single  character  in  common  with  the 
Labrador  or  Nofian  series  as  defined  by  Dr.  T.  Sterry  Hunt 
and  the  Canadian  Geological  Survey.  Dr.  Hunt,  also,  in 
1870,  in  consequence  of  finding  the  Eozoon  Canadense  in 
the  limestone  at  Newbury,  referred  a  portion  of  this  series  to 
the  Laurentian  age.  But  it  will  be  shown  in  the  sequel,  that 
all  the  rocks  within  the  area  described  belong  to  one  and  the 
same  lithological  and  stratigraphical  series,  the  characters  of 
which  stamp  it  as  undoubtedly  Huronian. 

A  glance  at  the  map  will  show  that  the  attempt  to  map 
this  formation  lithologically,  i.  e.,  to  show  the  distribution  of 
its  various  lithological  members,  has  been  attended  by  mod- 
erate success.  More  might  have  been  accomplished  with  a 
longer  time  for  exploration;  yet  much  must  ever  remain 
undone,  on  account  of  the  great  extent  to  which  the  rocks 
are,  in  some  districts,  concealed  by  superficial  deposits.  A 
special  color  has  been  used  to  represent  the  general  Huron- 
ian formation,  wherever  the  particular  lithological  represent- 
ative is  not  known,  but  the  probabilities  are  great  that  the 
rock,  whatever  it  is,  belongs  to  this  age. 

The  Huronian  series  in  Massachusetts  is  composed  of  the 
following  rocks,  or  groups  of  rocks :  hornblendic  granite, 
felsite,  diorite,  stratified  rocks,  and  limestone. 

HORNBLENDIC  GRANITE. — The  typical  hornblendic  granite 
of  this  region,  as  shown  at  the  quarries  in  Quincy,  Rockport, 
and  other  places,  is  a  coarsely  crystalline  aggregate  of  ortho- 
clase,  quartz,  and  hornblende.  The  hornblende  is  usually 
small  in  amount,  and  the  rock  frequently  passes,  through  the 
disappearance  of  hornblende,  into  binary  granite.  The  feld- 
spar is  usually  grayish  or  bluish,  though  red  and  green  tints 
are  frequently  met  with.  It  is  worthy  of  note  that  this  rock 
is  destitute  of  mica,  or  at  least  its  presence  is  a  very  rare 
occurrence.  This  typical  granite  frequently  passes  into  finer 


14 


grained  varieties,  which,  when  hornblende  is  absent,  pass 
through  eurite  into  felsite:  The  more  hornblendic  varieties 
are  usually  fine  grained ;  and  the  increase  of  hornblende  is 
attended  by  a  diminution  of  quartz,  so  that  the  rock  exhibits, 
through  the  entire  absence  of  quartz,  frequent  passages  into 
diorite.  That  these  granites  are  mainly  exotic,  can  scarcely 
be  questioned,  for  we  have  seldom  far  to  look,  to  find,  in  the 
form  of  enclosed  angular  fragments  of  clearly  stratified  rocks, 
evidence  of  their  extravasation ;  and  near  the  boundaries 
of  the  granites  we  usually  find  them  cutting  the  adjoining 
rock,  especially  if  that  is  stratified,  in  a  manner  incompatible 
with  any  theory  that  would  regard  them,  in  their  present 
condition,  as  indigenous  or  endogenous.  Nevertheless  it  is 
doubtless  true,  as  suggested  by  Prof.  N.  S.  Shaler,  and  later 
by  Mr.  T.  T.  Bouve,  that  these  granites  have  been  derived 
from  sedimentary  rocks,  and  have  simply  reached  the  final 
term  in  the  metamorphic  process] —  igneous,  or  more  prob- 
ably, igneo-aqueous  fusion.  A  glance  at  the  Centennial 
Map  suffices  to  show,  that  the  geographical  distribution  of 
these  rocks  is  in  harmony  with  the  theory  that  they  are 
extravasated  masses,  for  they  are  seen  to  occur  among  the 
other  members  of  the  Huronian  series,  in  a  manner  wholly 
irregular.  They  do  not  occur  in  continuous,  well-defined 
areas,  but  in  isolated  patches  with  boundaries  difficult  to 
define,  and  such  as  could  be  accounted  for  only  on  the  sup- 
position that  the  rock  has  been  more  or  less  fluent,  and  has 
experienced  some  extravasation.  The  large  area  colored  as 
granite  in  southern  Norfolk  and  northern  Plymouth  counties, 
would  appear  to  be  an  exception  to  the  above  statement; 
but  it  should  be  said  of  this  area  that  it  is  largely  covered 
by  drift  material,  especially  toward  the  southern  border,  and 
comparatively  few  observations  of  the  rocks  have  been  made. 
Slates  are  said  to  occur  in  Abington,  and,  although,  with  this 
exception,  no  rock  has  been  observed  in  this  area,  within 
the  writer's  knowledge,  but  granite,  it  is  quite  possible  that 
this  broad  area  includes  more  or  less  extensive  patches  of 


15 

other  rocks.  A  more  detailed  map,  such  as  it  is  hoped  the 
student  of  the  near  future  will  construct,  would  show  the 
boundaries  of  these  areas  of  granite  to  be  much  more  com- 
plex and  irregular  than  they  are  here  represented. 

A  further  inspection  of  the  map  will  make  it  evident  that 
these  granites  are  not  without  some  system  in  their  madness, 
for  the  most  of  the  granitic  areas  north  of  a  line  running 
from  Natick  to  tf eponset,  will  be  seen  to  be  arranged  along 
a  line  having  a  direction  about  N.  50°  E.,  forming  a  discon- 
tinuous band  of  variable  width,  which  extends  from  Natick 
to  Rockport  on  Cape  Ann ;  and  we  readily  find  in  this  wall 
of  granite,  probably  the  most  resistant  rock  of  this  region,  a 
sufficient  cause  for  the  existence  of  this  prominent  headland. 
The  slates  and  conglomerates,  in  Newton  and  Watertown, 
are  probably  underlaid  by  granite  belonging  to  this  range, 
which  accounts  for  the  discontinuity  of  the  range  here. 

Between  this  belt  of  granite  and  the  large  granitic  area 
on  the  south,  are  several  large,  irregular  areas  of  granite, 
including  the  Blue  Hill  range;  and  it  is  probable  that  part 
at  least  of  the  large  area  marked  on  the  map  as  general 
Huronian,  in  Medway,  Medfield  and  Franklin,  now  exten 
sively  drift-covered,  is  underlaid  by  granite.  I  have  tried  in 
vain  to  detect  any  system  or  order  in  the  arrangement  of 
these  masses,  and  can  only  say  that  their  present  complicated 
and  apparently  systemless  disposition,  is  probably  due  to  two 
forces  or  tendencies  operating  to  arrange  the  granites  in  two 
sets  of  parallel  bands ;  one  set  coinciding  in  direction  with 
the  prevailing  strike  of  the  region,  and  the  Natick  and  Rock- 
port  belt,  north-east  and  south-west;  and  the  other  set  hav- 
ing an  east  and  west  trend.1 

On  the  map,  in  the  northern  half  of  Essex  county,  I  have 
delineated  three  areas  of  granite,  in  Rowley,  Ipswich  and 

1  The  outlines  of  the  granites  in  Dedham,  Dover,  Sherburne,  etc.,  as  well  as  of 
the  other  rocks  of  that  region,  are  largely  due  to  the  observations  of  Mr.  F.  W. 
Very.  Valuable  data  concerning  the  distribution  of  these  rocks  near  Boston,  ha 
alsojseen  received  from  Mr.^W.  W.  Dodge. 


16 


Topsfield.  The  Topsfield  rock  disintegrates  readily,  and  is 
known  as  the  "  rotten  rock."  These  three  patches  are  widely 
separated,  yet,  inasmuch  as  no  other  rocks  have,  within  my 
knowledge,  been  observed  between  them,  and  since  the  char- 
acter of  the  drift  indicates  the  existence  of  considerable 
granite  in  this  region,  I  feel  justified  in  concluding  that  we 
have  here  a  broad  belt  of  granite,  beginning  at  the  shore  in 
Ipswich  and  extending  thence  south-westerly,  gradually  dis- 
appearing among  the  diorites  in  Boxford  and  Middleton ; 
but  I  have  deemed  it  best  not  to  map  beyond  the  known 
facts.  Toward  the  northern  end  of  Essex  county,  in  Newbury 
and  Salisbury,  are  other  areas  of  granite,  which,  for  the  most 
part,  are  small  and  are  known  to  lie  in  ranges  parallel  with 
the  stratified  rocks  of  that  district,  i.e.,  north-east  and  south- 
west. But,  since  they  have  been  only  imperfectly  worked 
out,  and  their  number  and  boundaries  remain  to  be  deter- 
mined, I  have  not  delineated  them  on  the  map.  To  avoid 
confusion  in  the  mind  of  the  reader,  it  should  be  stated  that, 
in  the  preceding  account  of  the  distribution  of  the  granite, 
no  notice  has  been  taken  of  the  very  hornblendic,  fine 
grained  granites,  which  frequently  pass  into  diorites,  and  are 
so  involved  with  the  diorites  in  their  origin  and  distribution, 
as  to  be  inseparable  from  them.  They  are  in  fact  more 
truly  diorites  than  granites. 

FELSITE. — Porphyry  is  a  much  abused  word,  which,  like 
syenite  (by  syenite  is  not  meant  the  aggregate  of  orthoclase 
and  hornblende  to  which  this  term  has  recently  been  ap- 
plied), has  outlived  its  usefulness,  and  should,  in  the  opinion 
of  the  best  geologists,  be  allowed  to  become  obsolete  in  its 
substantive  use,  as  a  geological  term.  Much  of  the  rock  in 
this  region  to  which  the  term  porphyry  is  applied,  is  not 
even  porphyritic;  showing  into  what  logical  errors  we  are 
led  when  we  choose  for  the  basis  of  a  lithological  name  a 
property  common  to  many  rocks,  and  which  cannot  be 
correlated  with  any  particular  chemical  constitution.  It  is 
not  intended  to  include  under  the  term  felsite,  either  in  this 


17 


description  or  on  the  map,  the  felsite  which  is  clearly  inter- 
stratified  with  quartzite,  hornblende  slate,  &c.  This  division 
is  entirely  arbitrary,  but  is  warranted  by  the  impracticability 
of  separating  the  different  members  of  what  may  be  called 
the  stratified  group.  To  avoid  confusion  I  will  speak  of  the 
felsite  of  the  stratified  group  as  stratified  felsite.  It  is  not 
meant,  as  will  be  seen  farther  on,  that  all  the  stratified 
felsites  belong  to  the  stratified  group  of  rocks,  but  only  such 
as  are  clearly  intercalated  with  that  group.  The  term  felsite, 
even  as  thus  limited,  includes  in  this  region  numerous  vari- 
eties, which  will  be  briefly  described. 

(a)  The  most  abundant  variety  is  the  porphyritic  felsite  of 
Lynn,  Saugus,  Maiden  and  Melrose.  It  consists  of  a  quartzo- 
feldspathic  base,  with  interspersed  crystals  of  feldspar.  It  is 
usually  more  or  less,  but  seldom  very  porphyritic.  Its  colors 
are  brownish  purple,  reddish  brown,  gray,  varying  through 
different  shades  of  drab  to  black.  In  general  it  is  a  structure- 
less rock,  showing  no  traces  of  bedding ;  but  at  Dungeon 
Rock  in  Lynn,  it  is  distinctly  stratified,  a  dense,  black  vari- 
ety being  interstratified  with  a  crystalline  dioritic  variety;* 
and  going  northward  in  Melrose,  the  porphyritic  character 
gradually  disappears,  the  felsite  becomes  more  siliceous,  and 
gradually  becomes  interstratified  with  quartzite  and  horn- 
blende slates.  The  transition  is  so  gradual  that  it  is  impossi- 
ble to  define  the  boundary  between  the  stratified  and  unstrat- 
ified  felsites,  which  proves  there  is  no  break,  no  natural 
division  here.  In  Melrose  and  Maiden,  and  at  other  points 
the  porphyritic  felsites  exhibit  frequent  local  passages  into 
granite  and  diorite.  It  seems  probable  that  considerable 
portions  of  this  rock  have  been  in  a  more  or  less  fluent  state  > 
this  can  scarcely  be  doubted  on  the  west  shore  of  Wenuchus 
Lake  in  Lynn,  where  the  tongue  of  granite  penetrates  the 
felsite ;  and  on  Marblehead  Neck  and  the  neighboring 
islands,  there  is  abundant  evidence  of  the  softening  and 
extravasation  of  portions  of  the  rock.  Notably  on  Marble- 
head  Neck,  als^  on  Red  Rock  in  Lynn,  and  at  the  Pirate's 
3 


18 


Glen  in  Saugus,  and,  perhaps,  at  other  points,  this  felsite 
exhibits  traces  of  a  conglomerate  origin.  This  variety  of 
felsite  is  seldom  found  beyond  the  localities  mentioned;  it 
has,  however,  been  observed  in  Hyde  Park,  near  the  corner 
of  Huntington  Avenue  and  River  Street,  and  at  some  points 
in  West  Dedham  and  Dover. 

(b)  Passing  westward  from  Maiden,  the  porphyritic  felsite, 
(a),  graduates  into  a  non-porphyritic,  homogeneous,  struc- 
tureless variety,  which   includes  all  the  felsite  of  Medford, 
the  felsite  of  West  Needham,  most  of  the  felsite  of  Natick, 
Dover,  and  Dedham,  and  a  small  portion  of  that  occurring  in 
Hyde  Park  and  Mattapan.     This  typical  felsite  is  usually  of 
a  grayish  color,  varying  from  white  to  black,  but  greenish 
shades  are  not  uncommon.  As  we  should  expect,  it  frequently 
passes  into  the  porphyritic  variety,  and,  like  that,  also  proves, 
by  its  frequent  transitions  to  granite  and  diorite,  the  intimate 
causal  connection  of  these  various  rocks.     This  variety  in- 
cludes the  so-called  "  Saugus  Jasper,"  a  bright  red,  homo- 
geneous felsite. 

There  are  in  this  region  two  principal  varieties  of  "banded" 
or  laminated  felsite,  which  differ  widely  in  their  origins:  first, 
that  in  which  the  banding  is  due  to  a  conglomerate  origin, 
having  been  produced  by  a  flattening  of  the  pebbles  of  the 
conglomerate,  and  which  is  described  farther  on ;  and  second, 

(c)  The  much  more   abundant    and   widely   distributed 
variety  in  which  the  banding  represents  the    original  bed- 
ding of  the  rock.      In  this  variety,  which  is   usually  of  a 
reddish   brown  color,  though  frequently  of  different  shades 
of  gray  and  sometimes  white,  the  banded  structure  is  usually, 
where  the  rock  has  not  been  greatly  disturbed,  very  uniform, 
regular  and  continuous.    The  banding  commonly  results  from 
the   interlamination    of  thin   layers  of  qunrtzose    and   fcld- 
spathic   materials.      The   thickness   of   the   lamina?    usually 
varies  from   a   mere   line  to  one-sixteenth   of  an  inch,  and 
seldom  exceeds  one-eighth  of  an  inch.      That  this  banding 
really  represents  stratification  is  proved  by  tlxe  regularity  and 


19 

continuity  of  the  bands,  since  a  banded  structure  due  to  the 
flattening  of  the  pebbles  of  a  conglomerate  would  necessarily 
exhibit  little  uniformity  in  the  thickness  of  the  lamina,  and 
I  find  it  difficult  to  conceive  of  pebbles  flattened  to  such  an 
extent  as  to  produce  continuous  layers  of  uniform  thickness 
and  yards  in  extent ;  and  it  is  also  proved  by  the  constancy 
of  their  strike  and  dip.  The  felsite  at  Kent's  Island  and 
other  points  in  Newbury,  is  mainly  of  the  banded  variety. 
It  has  a  reddish  color,  and,  though  showing  great  disturb- 
ance, the  bands  dip  with  considerable  regularity  to  the 
north  and  north-west ;  it  seems  to  pass  into  the  non-porphy- 
ritic  felsite  (b).  The  banded  felsite  is  occasionally  slightly 
porphyritic,  as  on  the  north-west  side  of  Marblehead  Neck, 
where  it  is  of  a  grayish  color  with  bands  from  one-sixteenth 
to  one-eighth  of  an  inch  thick,  and  dipping  steeply  to  the 
south-east.  A  small  amount  of  banded  felsite,  similar  to 
that  in  Newbury,  occurs  in  West  Dedham,  intercalated  with 
the  felsitic  breccia,  soon  to  be  described.  Banded  felsites  also 
occur  in  Hyde  Park,  east  of  the  Clarendon  Hills  Station,  in 
Mattapan,  and  in  Milton  immediately  south  of  the  Neponset. 
In  Mattapan  it  passes  into  the  closely  related  variety,  not 
logically  separable  from  it,  in  which  we  seem  to  have  the 
stratification  without  the  banding ;  there  appear  to  be  no 
quartzose  layers,  but  this  may  due  to  the  extreme  thinness 
of  the  bedding,  which  can  be  detected  only  on  weathered 
surfaces.  The  strike  is  K  E.-S.  W.  Mr.  W.  W.  Jacques 
reports  the  occurrence  of  banded  felsite  on  the  west  end 
of  the  large  hill  in  Quincy,  west  of  Willard  St.,  and  one  and 
a  half  miles  south-west  of  West  Quincy  Village. 

(d)  One  important  variety  of  felsite  yet  remains  unde- 
scribed.  It  is  porphyritic  with  feldspar  crystals  and  contains, 
in  addition,  numerous  grains  of  translucent  quartz,  which 
cause  some  portions  of  it,  where  the  quartz  and  feldspar 
crystals  are  abundant,  to  resemble  granite ;  and  it  may  be 
properly  called  the  granitoid  felsite.  It  occurs  extensively  in 
Needham,  and  the  southern  half  of  the  Blue  Hill  region* 


20 


including  all  the  higher  summits,  is  composed  of  it.  Through- 
out the  latter  district  the  base  is  of  a  dull  brown  color,  and 
the  rock  is  of  a  very  uniform  character,  but  toward  the  east, 
north  and  west,  it  undoubtedly  becomes  more  crystalline, 
more  granitic,  and  passes  into  the  Quincy  granite,  and  the 
granite  of  Dedham.  This  is  the  opinion  expressed  by  Prof. 
Edward  Hitchcock  nearly  forty  years  ago,  and  most  observers 
since  then  have  taken  the  same  view  of  the  matter.  In 
Keedham  the  base  is  of  a  white  and  greenish  white  color, 
and  contains  more  crystalline  quartz  than  feldspar.  It 
passes  into  fine  grained  granite  toward  the  east  and  south. 
Good  examples  of  the  granitoid  felsite,  of  small  extent, 
occur  on  Lowell's  Island  and  the  north-west  shore  of 
Marblehead. 

(e)  The  felsitic  breccia,  which  occurs  at  many  points  in 
this  regii  n,  should  be  mentioned  in  this  connection.  It 
occurs  in  Hyde  Park  and  Mattapan,  where  its  colors  are 
white,  purplish,  gray,  pinkish  and  bright  red.  It  varies 
greatly  in  texture  here,  sometimes  containing  angular  masses 
several  feet  in  extent;  and  also  exhibits  extensive  flattening 
of  the  pebbles,  producing  a  sort  of  pseudo-stratification. 
The  breccia  occurs  in  West  Dedham,  near  the  Oven  Mouth. 
Here  the  predominating  colors  are  red  and  black ;  and  the 
pebbles  do  not  show  alteration ;  but  the  breccia  appears  to 
pass  into  felsite  through  a  gradual  diminution  of  the  sizes  of 
the  pebbles,  just  as  conglomerate  passes  into  slate. 

On  Marblehead  Neck  the  breccia,  which  is  here  more 
properly  a  conglomerate,  becomes  at  some  points,  epecially 
on  Lowell's  Island,  a  coarse,  gritty,  feld spathic  sandstone, 
and  both  the  conglomerate  and  sandstone  pass  into  compact 
felsite,  the  former  in  two  distinct  ways,  which,  although 
observable  at  several  points  in  this  region,  are  best  exempli- 
fied here.  These  two  modes  of  metamorphism  are  :  (1)  By 
a  blending  together  of  the  pebbles  and  paste,  whereby  the 
outlines  of  the  former  are  lost,  or,  when  the  process  is  not 
complete,  can  only  be  seen  on  weathered  surfaces.  (2)  By 


21 


a  flattening  or  drawing  out  of  the  pebbles  into  thin  lentic- 
ular laminae,  which,  more  or  less  coalescing  at  their  edges 
and  lying  in  parallel  planes,  produce  a  stratified  appearance 
in  the  rock,  and  give  rise  to  a  laminated  or  banded  structure 
closely  resembling  that  already  described,  due  to  original 
sedimentation. 

That  compact  felsites  have  been  derived  from  genuine 
conglomerate  in  the  ways  just  indicated,  has  been  -fully  dem- 
onstrated by  Mr.  T.  T.  Bouve,  in  Hingham,  and  Prof. 
Alpheus  Hyatt,  on  Marblehead  Neck.  Their  views  on  the 
subject,  together  with  the  main  facts,  have  been  published  in 
the  Proceedings  of  the  Boston  Society  of  Natural  History, 
vol.  XVIII,  p.  217.  A  discussion  of  .this  subject  may  also 
be  found  in  the  paper  referred  to  in  the  foot  note  on  p.  5,  in 
which  it  is  further  shown  that  the  massive,  structureless 
felsites,  (a)  and  (£),  have  probably  been  largely  derived  from 
massive,  obscurely  stratified,  feldspathic  slates,  while  the 
normally  banded  felsite,  (c),  represents  a  finely  and  dis- 
tinctly stratified  slate. 

An  inspection  of  the  map  shows,  that  the  felsite  on 
Marblehead  Neck,  and  the  adjacent  islands,  forms  an  area 
five  miles  long  in  a  north-east  and  south-west  direction ;  a 
broad,  continuous  belt  of  felsite  extending,  in  the  same  direc- 
tion, from  Red  Rock  in  Lynn,  to  Arlington;  and  a  nar- 
row band  of  the  same  rock  stretching  south-westerly  from 
the  north-east  corner  of  Needham  to  South  Natick.  Thus  we 
have  an  interrupted  belt  of  felsite  reaching  from  Half  Way 
Rock  to  South  Natick,  a  distance  of  33  miles.  An  out- 
lying patch  of  felsite  is  said  to  occur  on  the  Manchester 
shore,  on  the  line  of  this  belt.  It  is  believed  that  this  belt 
of  felsites  is  really  much  wider  than  here  indicated ;  for  all 
the  crystallines  occurring  among  the  newer  rocks  between 
this  belt  on  the  north  of  Boston,  and  the  Quincy  granite 
range  on  the  south,  excepting  such  as  are  clearly  eruptive, 
are  felsites.  There  is  an  isolated  area  of  felsite  in  Chelsea, 
and  felsite  is  believed  to  occur  on  the  South  Shore,  in 


Hingham.  So  that,  if  the  newer  formations  and  water  were 
removed  from  Boston  Harbor,  and  vicinity,  we  should  prob- 
ably have  a  large,  unbroken  area  of  felsites,  trending  N".  E. 
-S.  W.,  and  vaiying  from  eight  to  twelve  miles  in  breadth. 
Toward  the  south-west,  in  Dedham,  Dover,  &c.,  the  felsites 
partake  of  the  complexity  which  appears  to  reign  in  that 
region. 

DIOBIT-E. — It  is  intended  to  include  here  only  the  eruptive 
diorites,  using  the  term  eruptive  in  a  general  sense  that  will 
include  all  rocks  that  have  been  fluent,  whether  extravasated 
or  not.  There  are  clearly  stratified  diorites  in  this  region, 
but  they  are  inseparable  from  the  other  stratified  members  of 
the  Huronian  series,  and  will  be  considered  as  forming  part 
of  the  stratified  group.  The  diorite,  like  the  granite,  varies 
greatly  in  texture  and  composition.  As  a  rule  it  is  quite 
fine  grained,  often  impalpably  fine,  and  never  attains  the 
degree  of  coarseness  common  with  the  granites.  In  compo- 
sition it  has  a  wider  range ;  as  already  stated,  it  passes,  by 
an  admixture  of  quartz,  into  fine  grained  hornblendic  gran- 
ite ;  and  it  is  no  less  prone,  by  losing  hornblende,  to  pass 
into  felsite.  Fine  examples  of  the  transition  between  diorite 
and  felsite  may  be  seen  in  Greenwood  and  Stoneham.  Near 
the  Greenwood  station  on  the  Boston  and,  Maine  R.  R.,  the 
diorite  contains  little  or  no  hornblende,  and,  but  for  its  more 
crystalline  appearance,  would  be  indistinguishable  from  fel- 
site. The  diorites  are  never  very  micaceous,  though  fre- 
quently containing  some  biotite.  The  biotite  usually  occurs 
in  the  more  crystalline  varieties  of  the  diorite.  The  areas 
colored  as  diorite  on  the  map,  though  mainly  diorite,  yet  em- 
brace a  great  amount  of  fine  grained  hornblendic  granite, 
which,  however,  is  frequently  little  more  than  a  quartzose 
diorite.  Any  observer  of  these  two  rocks  will  agree  with 
me  that  they  admit  of  neither  a  lithological  nor  a  geographical 
separation.  As  a  rule  they  are  both  eruptive,  and  over 
large  areas  they  have  been  extravasated  through  each  other 
so  extensively,  and  the  action  has  been  so  mutual,  that  the 


23 

complication  is  complete;  and  I  have  long  been  accustomed 
to  speak  of  them  as  "mixed  rocks";  and  I  know  now  of  no 
term  that  will  better  express  their  relations,  lithologically  or 
petrologically. 

The  large  area  in  Sharon,  Stoughton  and  Foxborough, 
colored  as  diorite,  may  include  some  small  areas  of  coarse 
granite,  but  is  mainly  of  the  dioritic  or  "mixed"  series. 
The  diorite  of  this  area  is  chiefly  very  fine  grained,  but  some 
quite  coarse  varieties  occur  in  the  hills  of  Sharon,  and  Mr. 
F.  W.  Very  has  observed  the  same  in  Foxborough.  The 
diorites  in  other  parts  of  Norfolk  county  are  mainly  fine 
grained  and  exhibit  frequent  passages  into  horablendic 
granite  and  felsite.  The  diorites  seem  to  obey,  in  their 
distribution,  the  same  general  law  as  the  granites  and  fel- 
sites;  and  the  diorite  areas  north  of  Boston  form  a  nearly 
continuous,  but  very  irregular  belt,  lying  parallel  with  and 
north  of  the  north-east  and  south-west  granite  range,  and 
extending  from  Beverly  to  Wayland.  *  In  the  north  part 
of  Essex  county,  north  and  west  of  the  Newbury  belt  of 
felsite,  there  is  much  diorite  occurring  among  the  stratified 
rocks  of  that  region ;  but  too  few  observations  have  been 
made  for  its  delineation  on  the  map. 

STRATIFIED  ROCKS. — While  describing  the  preceding  mem- 
bers of  this  formation,  it  has  been  pointed  out  that  they 
frequently  pass  into  each  other ;  and  it  would  seem  to  be 
nearly  an  established  fact  that  they  all  belong  to  one  and  the 
same  series;  but  a  study  of  the  stratified  group  greatly 
strengthens  the  evidence  in  favor  of  the  oneness  of  their 
origin;  for  here  we  find  them  all  combined  and  blended 
together.  As  already  stated,  we  find,  on  going  northward 
through  Melrose,  the  porphyritic  felsite,  (a),  gradually  be- 
coming less  porphyritic  and  assuming  a  stratified  appearance] 
North  of  Howard  Street  traces  of  stratification  are  common, 
though  porphyritic  felsite  occurs  as  far  north  as  Greenwood. 
North  of  Central  Brook,  in  Saugus,  the  felsites  are  chiefly 
stratified,  the  bedding  increasing  northward,  are  largely 


24 


quartzose,  passing  into  quartzite,  and  are  frequently  inter- 
stratified  with  hornblende  slate  and  stratified  diorite.  No 
observer,  who  has  been  over  this  ground,  can  doubt  that  these 
different  rocks  are  stratigraphically  inseparable.  Along  the 
eastern  border  of  this  area  of  stratified  rocks,  one  can  find 
beautiful  examples  of  the  passage  of  stratified  diorite  and 
hornblende  slate  into  the  eruptive  diorite  and  fine  grained 
hornblendic  granite.  There  is  nothing  abrupt  about  these 
transitions,  the  gradation  is  perfect.  The  stratified  rocks 
strike  N.  E.-S.  V.,  and  dip  very  steeply  to  the  N.  W. 
About  four  miles  farther  north-west,  in  Reading,  is  another 
area  of  stratified  rocks,  which  includes  the  varieties  pre- 
viously mentioned,  but  with  relatively  more  hornblende  slate 
and  diorite ;  and  some  of  the  felsite  is  slightly  micaceous. 
Here,  also,  we  find  the  rocks  shading  insensibly  into  eruptive 
diorite  and  granite.  The  strike  and  dip  are  the  same  as  in 
Saugus  and  Melros^e. 

On  the  Boston  and  Lowell  R.  R.,  near  the  North  Woburn 
line,  is  a  distinct  argillite  conformably  interstratified  with 
rocks  similar  to  those  just  described.  The  narrow  Huronian 
band,  extending  from  Concord  to  Westborough,  lies  on,  and 
parallel  with,  the  line  of  strike  of  the  stratified  rocks  in 
Reading  and  North  Woburn,  and  consists  of  stratified 
diorite  and  hornblende  slate,  with  a  small  amount  of  inter- 
calated mica  schist.  The  strike  is  parallel  with  the  trend  of 
the  band.  Between  Concord  and  the  Boston  and  Lowell 
R.  R.,  in  Burlington,  North  Lexington,  and  Lincoln,  there 
are  few  outcrops,  but  it  is  probable  the  rocks  are  chiefly 
stratified,  and  they  have  been  so  represented  on  previous 
maps.  At  Putnamville  in  Danvers,  and  Locustdale  in  Pea- 
body,  are  stratified  diorite,  hornblende  slate,  and  a  well 
marked  feldspathic  gneiss  containing  some  mica,  all  dipping 
steeply  to  the  N.  W.  The  character  of  the  drift  in  Wen- 
ham  indicates  a  wide  area  of  stratified  rocks  in  this  region  ; 
but  the  limits  are  not  known.  In  southern  Middleton  the 
drift  is  composed  largely  of  imperfect  gneisses,  passing  into 


25 


hornblendic  granite,  which  must  occur  here  in  situ.      The 
large  area  of  diorite  stretching  from  Stonehara  to  West  on 
includes  numerous  small  patches  of  stratified  rocks — horn- 
blende slate  and  petrosilex.     A  few  of  these  patches  have 
been  represented  on  the  map,  but  many  more  probably  re- 
main undiscovered.     They  are  usually  narrow,  parallel  with 
the  strike,  and  vary  in  length  from  a  few  rods  to  a  mile  or 
more.       Strike,    KE.-S.W. ;   dip,  steep  to    K  W.       They 
pass  frequently  into  the  enclosing  rock,  showing  that  they 
are  mere  remnants  of  the  stratified  group,  which  yet  pre- 
serve traces  of  the  structure   once  possessed  by  the  whole 
mass  of  the  rock.    Large  masses  of  the  hornblende  slate,  &c. , 
occur  enclosed  in  granite  at  Hospital  Point  on  the  Beverly 
shore,  and  in  Norian  rocks  on  the  north  side  of  Great  Misery . 
Irregular  fragments  and  masses  of  the  micaceous  petrosilex , 
hornblende  slate,  and  other  stratified  rocks,  are  occasionally 
found  enclosed  in  the  diorite  of  Swampscot,  Marblehead  and 
Salem;  and  the  diorite  itself  sometimes  appears  stratified' 
It   thus   becomes   evident  that   this   diorite,  the   so  called 
"  Salem  syenite,"  has,  like  the  diorites   elsewhere,  resulted 
from  the   extreme  metamorphism  of  the   stratified   group. 
The   mica  slate,  so   called,   enclosed  in   exotic  granite   on 
Marblehead    Neck,  belongs  to  this  stratified   group.1     The 
area  of  stratified  rocks  in  Waylaud  is  probably  much  larger 
than  represented  on  the  map.      They  are  distinct    gneisses 
which  belong  lithologically  to  the  Mont  Alban  rather  than 
the  Huronian  series ;  and  it  is  possible  the  Mont  Alban  bound- 
ary is  deflected  this  far  to  the  east,  but  more  probable  that 
the   rocks  are  Huronian.      Their  strike   is   N.-S.   and  dip  . 
vertical.      The  stratified  rocks  in  Sherburne  are  imperfect 
gneisses  and  stratified  diorites.     The  area  in  Dover  is  less, 
gneissic  and  more  felsitic.    It  is  deemed  probable,  judging  from 
the  character  of  the  drift,  that  the  Sherburne  area  should 
be  extended  southwesterly  into  Holliston.     Stratified  Huron- 

1  Much  assistance  in  mapping  the  rocks  in  this  portion  of  Essex  county  has  been 
derived  from  the  notes  of  Prof.  A.  Hyatt. 
4 


ian  rocks,  other  than  felsite,  are  of  rare  occurrence  soutli  of 
the  Charles  River,  and,  besides  the  area  in  Dover,  I  know  of 
but  one  locality  where  they  occur;  this  is  on  the  Cohasset 
shore,  between  Black  Rock  and  Little  Harbor,  where  there  is 
an  indistinct  feldspathic  gneiss,  poor  in  quartz.  In  the  north- 
ern part  of  Essex  county,  in  Newbury,  Newburyport  and  West 
Newbury,  the  stratified  rocks,  other  than  felsite  and  lime- 
stone, are  well  developed.  They  resemble  the  indigenous 
rocks  of  Reading,  but  are  less  felsitic,  and,  according  to  the 
observations  of  Mr.  C.  W.  Kernpton,  more  calcareous  and 
magnesian.  They  strike  K  E.-S.  W.  and  dip  to  the  N.  W. 
The  principal  ore  deposits  of  that  district  occur  in  these 
rocks.  Traces  of  bedding  appear  in  some  portions  of  the 
granite  in  Rowley. 

The  stratified  group  and  its  distribution  have  been  thus 
minutely  described  for  the  purpose  of  showing  its  intimate 
and  complex  petrological  relations  to  the  other  members  of 
the  series,  and  how  completely  it  binds  them  together  as  a 
lithological  unit.  I  have  already  indicated  that  much  of  the 
hornblendic  granite  has  been  derived  by  metamorphism  from 
felsite,  and,  in  the  paper  referred  to  on  p.  5,  I  have  shown 
that  this  is  the  probable  origin  of  all  the  Huronian  granites. 
In  the  same  paper,  it  is  demonstrated  that  the  most,  and 
probably  all,  of  the  felsites  of  this  region,  are  now,  or  were 
originally,  stratified,  and  may,  therefore,  be  logically  included 
in  the  stratified  group. 

I  have  now  described  all  the  rocks  occurring  within  the 
area  designated  as  Huronian,  except  the  limestone,  which  is 
small  in  amount  and  may  be  neglected  here :  and,  making  a 
resume  of  the  whole,  we  are  brought  to  the  conclusion, 
that  the  rocks  of  this  area  consisted  once,  speaking  gen- 
erally, of  stratified  felsite,  hornblende  slate  and  stratified 
diorite  only,  which  were  then,  and  are  even  now,  so  related 
stratigraphically,  and  exhibit  such  frequent  lithological  trans- 
itions, as  to  prove  them  to  be  members  of  one  and  the  same 
unbroken  series ;  and  that  the  felsites,  by  metamorphism,  have 
given  rise  to  granite,  the  hornblende  slates,  in  like  manner, 


27 


to  fine  grained  hornblendic  granite  approaching  diorite,  and 
the  stratified  diorite  to  eruptive  diorite. 

LIMESTONE. —  The  limestones,  or  more  correctly  dolomites, 
of  this  formation  are  not  abundant  in  this  region.  They  are 
usually  serpentinic  (ophiolite),  arid  are  seldom,  if  ever, 
coarsely  crystalline.  The  principal  deposits  occur  in  N"ew- 
bury  and  Lynnfield,  and  are  distinctly  stratified,  having  a 
N".  E.-S.  W.  strike,  and  dipping  to  the  N".  W. ;  but  small 
patches  are  also  known  in  Stoneham,  Natick  and  Walpole. 
In  Newbury  the  ophiolites  are  enclosed  in  diorite  and  gran- 
ite, and  in  Lynnfield,  though  the  relations  of  the  rocks  are 
much  obscured  by  drift,  the  association  appears  to  be  the 
same.  As  already  stated,  the  Eozoon  Canadense  has  been 
found  in  this  rock,  in  Newbury. 

All  the  stratified  rocks  of  this  formation,  north  and  west 
of  Boston,  including,  besides  the  so  called  stratified  group, 
the  dolomites  and  stratified  felsites,  dip,  with  few  and  unim- 
portant exceptions,  to  the  north-west.  And,  summarizing 
what  has  been  said  concerning  the  distribution  of  these 
rocks,  we  find,  proceeding  from  southeast  to  irorth-west,  the 
following  general  geographical  arrangement.  Underlying 
Boston  and  its  environs,  is  the  broad  belt  of  felsite  already 
described ;  and  to  the  north-west  occur  in  succession,  petro- 
silex,  hornblende  slate,  stratified  diorite,  and  rocks  more 
or  less  micaceous,  to  the  limits  of  the  formation.  And, 
since  the  limits  of  the  different  rocks  are  marked  by  grad- 
ual transitions,  which  precludes  the  existence  of  faults  of 
any  great  extent ;  and  since  there  are  no  apparent  repeti- 
tions of  the  different  rocks,  which  precludes  the  exist- 
ence of  any  considerable  folds  of  the  strata ;  we  are  forced 
to  the  conclusion  that  the  geographical  arrangement  cor- 
responds with  the  stratigraphical  succession ;  and  the  rocks 
to  the  south-east  must  underlie  those  to  the  north-west — the 
felsites  must  be  older  than  the  hornblende  slates  and  diorites. 
South  of  the  felsite  is  the  broad  area  of  granite  in  Norfolk 
and  Plymouth  counties;  and  since  the  granites  are  more 
intimately  associated  with  the  felsites  than  any  other  of  the 


28 


stratified  rocks,  and  are  probably  derived  from  them;  and 
since  the  exotic  granites  occur  as  eruptives  through  all  the 
stratified  rocks ;  the  inference  is  plain  that  the  granites  be- 
long normally  at  the  bottom  of  the  series,  and  may  be  taken 
to  represent  the  lower  portions  of  the  felsite. 

The  evidence  supporting  these  conclusions  is  given  in 
detail,  accompanied  by  a  general  section  illustrating  the 
stratigraphy  of  this  region,  in  the  paper  referred  to  on  p.  5 ; 
and,  in  the  same  paper,  attention  is  called  to  the  interesting 
fact  that  in  the  northern  part  of  Essex  county,  we  find  sub- 
stantially the  same  geographical  arrangement,  and  corre- 
sponding stratigraphical  succession,  as  that  just  described,  but 
on  a  smaller  scale.  There,  as  farther  south,  the  stratified 
crystallines  all  dip  to  the  north-west;  and  we  have,  on  the 
south-east,  the  granite  of  Ipswich  and  Rowley,  succeeded  by 
the  felsite  of  Kent's  Island  and  the  River  Parker,  and  this 
by  the  eruptive  diorite,  hornblende  slate,  and  stratified  dior- 
ite,  between  the  River  Parker  and  the  Merrimac.  In  the 
paper  so  often  referred  to,  it  is  shown,  after  a  consideration 
of  many  details  omitted  here,  that  this  grand  repetition  of 
the  strata  necessitates  the  existence  of  a  gigantic  fault,  ex- 
tending south-westerly  from  the  shore  in  Ipswich,  and  having 
the  down-throw  on  the  south-east.  The  distribution  of  the 
rocks  toward  the  south-west  indicates  that  the  fault  wanes  in 
that  direction,  the  vertical  displacement  being  insufficient  to 
bring  up  the  granite  beyond  Middleton  ;  but  it  is  conceived 
that  the  line  of  fracture  extends  along  the  southern  border 
of  the  Mont  Alban  formation,  through  Concord,  and  does 
not  finally  die  out  before  reaching  Westborough.  And  it  is 
further  believed,  that  we  may  find  in  this  great  dislocation  a 
sufficient  cause  for  the  existence  of  the  peculiar  Huronian 
peninsula,  which,  as  already  stated,  extends  south-westerly 
from  Concord,  penetrating  the  Mont  Alban  formation.  This 
is  the  principal  fault  of  this  region ;  but  many  minor  fractures 
exist,  some  of  which  are  minutely  described  in  the  paper 
referred  to. 


The  oneness  of  this  formation  has  been  established ;  it  is 
proved  by  its  petrological,  its  lithological,  and,  as  I  have  else- 
where shown,  its  chemical  characters.  I  have  pointed  out, 
in  the  preceding  pages,  that  it  distinctly  overlies  the  Norian 
rocks ;  and  it  no  less  clearly  underlies  the  Mont  Alban.  It 
exhibits  much  greater  disturbance  than  the  Mont  Alban,  but 
less  than  the  Norian;  and  near  the  common  boundaries  of 
this  series  and  the  Mont  Alban,  the  gneiss  of  the  latter 
is  cut  by  the  eruptive  granite  and  diorite  of  the  former. 
And  since  this  series  so  closely  resembles,  in  its  internal 
characters  and  its  external  relations,  the  Huronian  of  other 
regions,  we  are  bound  to  conclude  that  it  is  the  Huronian. 

MONT  ALBAN. 

The  rocks  referred  to  the  Mont  Alban  age  cover  a  wide 
area  in  Massachusetts,  forming  the  greater  portion  of  the 
State.  They  extend  uninterruptedly  from  the  western 
boundary  of  the  Huronian  formation  to  the  Triassic  sand- 
stones of  the  Connecticut  Valley,  and  spread  over  most  of 
the  area  between  these  Mesozoic  rocks  and  the  Hoosuc 
Mountains.  They  are  also  wide-spread  in  the  southern  por- 
tions of  Bristol  and  Plymouth  counties,  about  the  shores  of 
Buzzard's  Bay.  The  Mont  Alban  areas  on  the  Centennial 
Map  comprise,  with  few  exceptions,  the  areas  marked  as 
"granite,"  "gneiss,"  "mica  slate,"  "argillite,"  "metamorphic 
slate,"  and  "  quartzite,"  on  the  geological  map  of  Prof.  Ed- 
ward Hitchcock.  They  also  exhibit  a  general  correspond- 
ence with  the  areas  marked  as  "White  Mountain  group," 
"  granite,"  "  Merrimac  schists,"  "  calciferous  mica  schist,"  and 
"  St.  John's  group,"  on  the  map  of  Prof.  C.  H.  Hitchcock,  in 
Walling's  Atlas  of  Massachusetts,  1871.  The  prevailing 
strike  of  the  rocks  of  this  formation  is,  stated  generally, 
N.-S.;  but,  east  of  Worcester,  it  approximates  to  N.E.- 
S.W.,  showing  a  tendency  to  parallelism  with  the  strike  of 
the  Huronian  beds. 

The  principal  lithological  constituents  of  this  formation 
:ire  granite,  gneiss,  mica  slate,  argillite,  and  limestone.     In 


30 


the  western  portion  of  the  State,  there  are  bands  of  horn 
blende  slate  and  stratified  diorite  included  in  the  Mont  Al- 
ban  series,  which,  though  usually  delineated  on  geological 
maps,  are  so  intimately  associated  with  the  gneiss  and  mica 
slate,  that  they  are  regarded  as  logically  inseparable  from 
them. 

GRANITE. — The  granites  are  of  two,  probably  three,  dis- 
tinct kinds, —  distinct  in  origin,  but  often  difficult  to  distin- 
guish with  the  eye.  They  are  exotic,  indigenous,  and  en- 
dogenous. The  exotic  or  eruptive  granite  is  of  most  import-, 
ance.  It  is  usually  whitish  or  gray, —  seldom  red  or  greenishr 
as  the  Huronian  granite  frequently  is ;  and  it  differs  furthe- 
from  the  Huronian  granite,  in  that  it  is  always  more  or  less 
micaceous,  and  seldom  contains  any  hornblende.  It  fre- 
quently passes  into  the  indigenous  variety,  which  is  nothing 
more  than  a  coarse,  granitoid,  apparently  structureless  gneiss. 
There  can  be  no  doubt  that  the  exotic  granite  has  resulted 
from  the  extreme  metamorphism  of  portions  of  the  gneiss ; 
and  hence  the  indigenous  and  exotic  varieties  are  essentially 
the  same,  and  I  shall  not  attempt  to  distinguish  them  here. 
Both  these  varieties  are  sometimes  porphyritic  with  crystals 
of  feldspar,  which  are  occasionally  very  large. 

The  distribution  of  these  granites  is  well  shown  on  the 
map.  In  the  western  half  of  the  State,  the  larger  areas  are 
found  along  the  borders  of  the  Triassic  sandstone,  where  the 
granites  occur  cutting  mica  slate  and  gneiss.  The  area  in 
Williamsburg,  Goshen,  Westhampton,  and  Northampton, 
and  also  that  east  of  the  Connecticut,  in  Amherst,  though 
marked  as  wholly  granitic,  include,  according  to  the  obser- 
vations of  Dr.  Hitchcock,  much  mica  slate,  which  is  associ- 
ated with  the  granite  in  such  a  manner  as  to  prove  the  ex- 
otic nature  of  the  latter.  On  the  Centennial  Map,  I  have 
colored  as  Mont  Alban  granite  the  two  areas  —  one  in  Lud- 
low  and  Belchertown,  and  the  other  in  Northampton  and 
Hatfield — marked  on  all  previous  maps  as  "syenite"  (horn- 
blendic  granite).  My  reasons  for  this  course  are :  (1)  These 


syenites,  so  called,  frequently  contain  nearly  or  quite  as  much 
mica  as  hornblende,  and  closely  resemble  portions  of  the  ad- 
joining granites.  They  are  really  hornblendo-micaceous  or 
quarfenary  granites,  and  bear  but  little  likeness  to  the  horn- 
blendic  granites  of  the  Huronian  formation.  (2)  Prof. 
Edward  Hitchcock,  on  his  revised  map  of  1844,  represented 
a  small  patch  of  granite  enclosed  in  the  Ludlow  and  Belcher- 
town  area  of  syenite.  (3)  Hornblendic  gneisses,  hornblende 
slates,  and  even  pure  hornblende  rock,  certainly  occur  in  the 
Mont  Alban  formation,  and  form  a  part  of  the  same ;  and, 
since  the  granites  have  resulted  from  the  metamorphism  of 
the  gneisses,  I  can  see  no  reason  why  the  Mont  Alban  gran- 
ites may  not  sometime  be  hornblendic. 

In  the  eastern  portion  of  the  State,  the  most  interesting 
area  of  granite  is  the  long  but  narrow  belt  which  extends 
from  Worcester  to  Dracut.  It  will  be  found  fully  described 
in  the  report  of  Mr.  L.  S.  Burbank,  which  accompanies  this. 
The  extended  and  little  known  area  of  granite  lying  north 
of  the  gneiss  in  Plymouth  and  Bristol  counties  is  referred  to 
this  age,  on  account  of  its  intimate  relations  to  the  gneiss, 
and  its  unlikeness  to  the  Huronian  granites.  The  granites 
cutting  the  Carboniferous  and  Primordial  slates  near  New- 
port are  also  referred  to  this  horizon.  They  resemble,  in 
some  respects,  the  quaternary  granites,  just  described,  of  the 
Connecticut  valley. 

The  endogenous  granites,  which  have  not  a  distinctive 
color  on  the  map,  are  usually  coarser  than  the  preceding,  and 
more  distinctly  crystalline;  never  exhibiting  traces  of  schist- 
osity,  or  genuine  stratification.  They,  however,  sometimes 
possess  the  banded  appearance  due  to  their  mode  of  deposi- 
tion, which  may  be  readily  mistaken  for  stratification,  since 
the  veins  are  frequently  of  a  lenticular  form,  usually  occur 
in  gneisses  of  similar  composition,  and  conform  in  trend  and 
dip  with  the  bedding  of  the  enclosing  rock.  These  gpanites 
are,  as  Dr.  T.  Sterry  Hunt  has  shown  to  be  the  case  with  en- 
dogenous Mont  Alban  granites  generally,  characterized  by 


32 


the  abundance  and  variety  of  the  accessory  minerals  which 
they  contain,  though  appearing  to  be  poorer  in  this  respect 
than  the  endogenous  granites  of  some  other  regions. 

GNEISS. — This  is  the  most  abundant  rock  in  Massachu- 
setts, forming  nearly  one-half  the  State.  Accurate  and  com- 
prehensive descriptions  of  it  may  be  found  in  Prof.  Edward 
Hitchcock's  final  report  on  the  Geology  of  the  State;  and  I 
will  only  add  here,  that  its  general  lithological  and  petro- 
logical  characters  stamp  it  as  unquestionably  Mont  Alban, 
rather  than  Laurentian,  gneiss.  Its  distribution,  as  shown  oti 
the  Centennial  Map,  corresponds  very  closely  with  previous 
maps,  and  there  is  only  one  important  exception  to  note. 
This  is  in  the  eastern  part  of  Plymouth  county,  where,  it 
will  be  observed,  the  bands  of  gneiss  and  granite  have  been 
extended  eastward  to  the  shore.  This  disposition  of  the 
rocks  seems  justified  by  the  E.-W.  strike  of  the  gneiss  in 
this  region,  and  by  the  following  extract  from  the  final  report 
of  Prof.  Hitchcock,  just  referred  to:  "  On  my  former  map  I 
colored  a  deposit  of  granite,  connected  on  the  north  with 
that  just  described,  and  extending  to  Brewster  on  Cape  Cod. 
I  did  this  because  a  ridge  of  considerable  elevation  extends 
down  the  Cape  to  Brewster,  and  many  boulders  of  granite 
are  found  of  great  size  upon  its  hills.  But  re-examination 
renders  it  probable  that  the  largest  and  most  abundant  of 
these  boulders  are  granitic  gneiss,  approaching  so  near  to  real 
granite,  as  easily  to  be  mistaken  for  it.  A  somewhat  similar 
rock  occurs  in  place  in  Rochester  and  New  Bedford ;  and  my 
present  impression  is,  that  probably  a .  ridge  of  this  gneiss, 
and  not  granite,  forms  the  axis  of  the  Cape." 

The  band  of  altered  slate  and  conglomerate  occurring  in 
Bellingham,  and  marked  on  the  map  of  Prof.  Edward 
Hitchcock  as  "metamorphic  slate,"  has,  on  the  Centennial 
Map,  been  merged  with  the  gneiss  of  that  region.  The  rea- 
sons for  this  are,  that  it  is  distinctly  interstratified  with  the 
gneiss,  and  exhibits  frequent  and  beautiful  passages  into 
gneiss  and  mica  slate,  indistinguishable  from  those  forming  a 


83 


large  part  of  the  great  Mont  Alban  formation.  I  am  inclined 
to  regard  it  as  essentially  a  part  of  the  gneiss,  and  to  see  in 
the  more  conglomeritic  portions  an  indication  of  the  con- 
dition of  a  large  part,  perhaps  all,  of  our  schistose  gneisses  at 
some  time  in  the  remote  past  —  a  small  sample  arrested  in  its 
development,  and  now  available  as  a  key  to  the  origin  of 
thousands  of  square  miles  of  crystalline  rocks. 

The  conglomerate  in  Harvard,  which  has  been  carefully 
studied  by  Mr.  L.  S.  Burbank,  and  will  be  found  described 
in  his  report,  though  placed  nearer  the  top  of  the  series,  and 
having,  HS  I  conceive,  no  stratigraphical  or  genetic  connec- 
tion with  the  great  body  of  the  gneisses,  exhibits  similar 
metamorphoses. 

MICA  SLATE. — Mica  slate  is,  next  to  gneiss,  the  most  abun- 
dant rock  in  the  State.  Its  characters  and  distribution  have 
been  well  described  by  Prof.  Edward  Hitchcock.  Some 
alterations  of  the  boundaries  have  been  made  in  the  Nashua 
and  Menimac  valleys,  which  are  explained  in  Mr.  Burbank's 
report.  This  rock  is  believed  to  generally  overlie,  conform- 
ably, the  gneiss.  Throughout  the  State  there  are  numerous 
bands  and  patches  of  mica  slate,  which  are  regarded  as  local 
variations  of  the  gneiss,  and  have  not  been  delineated  on  the 
map. 

ARGILLITE. — Argil lites  of  several  distinct  ages  occur  in 
Massachusetts.  To  the  Mont  Alban  age  are  referred  the  ar- 
gillites  of  the  Nashua  and  Connecticut  valleys  only.  The 
boundaries  of  this  rock  remain  substantially  as  they  appear 
on  Prof.  Hitchcock's  map.  It  unquestionably  overlies  the 
mica  slate  ;  and  its  lithological  and  petrol ogical  characters,  as 
described  by  Prof.  Edward  Hitchcock,  make  its  Mont  Alban  » 
age  extremely  probable.  The  same  is  true  of  the  mica  slate. 

The  narrow  strip  of  conglomerate  in  Harvard,  already  re- 
ferred to,  although  regarded  as  logically  inseparable  from  the 
argillite,  has  received  a  distinctive  color  on  the  map,  on  ac- 
count of  the  scientific  interest  which  attaches  to  it,  and  the 
accuracy  with  which  its  boundaries  have  been  worked  out. 
5 


LIMESTONE.— The  Mont  Alban  limestones  are  less  mag- 
nesian  than  those  of  the  Huronian  age,  and  are  rarely  ser- 
pentinic.  They  occur  in  numerous  small  lenticular  beds, 
usually  interstratified  with  gneiss  and  mica  slate.  The  de- 
posits east  of  the  Nashua  valley  are  regarded  by  Mr,  Bur- 
bank  as  partaking  mainly  of  the  nature  of  veins,  rather  than 
stratified  beds;  but,  west  of  the  Connecticut,  as  Prof. 
Hitchcock  has  pointed  out,  some  at  least  of  the  masses 
are  distinctly  and  regularly  stratified.  The  limestone  in 
Chelmsford  has  afforded  specimens  of  the  Eozoon  Cana- 
dense ;  and,  since  this  peculiar  structure  has  been  found  in 
Laurentian,  Huronian,  and  Mont  Alban  limestones,  those 
believing  in  its  organic  nature  must  admit  that  the  animal 
producing  it  had  a  very  long  range  in  time. 

Accompanying  the  report,  so  often  referred  to,  of  Prof. 
Edward  Hitchcock,  are  several  E.-W.  sections  of  the  State, 
which,  though  exhibiting  correctly  the  more  general  strati- 
graphical  features  of  the  Mont  Alban  formation,  leave  sev- 
eral important  points  in  great  obscurity.  Between  the 
western  boundary  of  the  Huronian  formation  and  the  mica 
slates  of  the  Nashua  valley,  in  the  northern  part  of  Middlesex 
county,  the  gneiss  all  dips  to  W.-N.  W. ;  the  amount  of  the 
dip  varying  from  50°  to  vertical.  There  are,  as  is  clearly 
shown  on  the  map,  three  N.-S.  bands  of  mica  slate  in  the 
Nashua  valley,  each  of  which  dips  steeply  to  the  west.  Be- 
tween the  first  and  second  of  these  belts,  reckoning  from  east 
to  west,  is  a  long  and  narrow  range  of  granite  and  granitoid 
gneiss ;  while  a  broad  band  of  argillite,  which  has  also 
steep  westerly  dips,  separates  the  second  and  third  belts  of 
,  mica  slate.  Between  the  first  range  of  mica  slate  and  the 
granite,  in  Harvard  and  Bolton,  lies  the  narrow  strip  of  con- 
glomerate and  argillite, — conglomerate  on  the  east  and 
argillite  on  the  west,  —  already  referred  to  as  the  Harvard 
conglomerate.  This,  like  all  the  preceding,  dips  to  the  west. 
West  of  the  third  range  of  mica  slate  is  a  broad  expanse  of 
gneiss,  which,  along  its  eastern  border,  dips  gently,  15°-25°, 


35 


to  the  east;  and  resembles,  in  its  essential  lithological  fea- 
tures, the  gneiss  of  Middlesex  county. 

Such  is  the  general  surface  arrangement  of  the  rocks  of  this 
region ;  and  this  arrangement  necessitates,  as  I  conceive,  the 
following  structure  below  the  surface:  The  gneisses  on  the 
eastern  and  western  sides  of  the  valley  are  stratigraphically 
the  same ;  the  three  belts  of  mica  slate  are  parts  of  one  and 
the  same  stratum;  and  the  strip  of  argillite  and  conglom^ 
erate  in  Harvard  and  Bolton  is  an  isolated  portion  of  the 
great  area  of  argillite  forming  the  centre  of  the  valley. 
This  valley  is  a  vast  synclinal,  with  its  axial  plane  dipping 
to  the  west,  and  having  a  grand  fault  on  either  side.  The 
broad  band  of  argillite  is  a  stratum  not  less  than  10,000  feet 
thick  folded  sharply  upon  itself,  and  forms  the  centre  of  the 
synclinal.  Enclosing  the  argillite  is  the  stratum  of  mica 
slate,  forming  the  second  and  third  bands  of  that  rock.  Be- 
tween the  third  band  of  mica  slate  and  the  gneiss  on  the 
west,  there  is,  as  already  indicated,  an  abrupt  break  in  the 
bedding :  the  slate  dips  steeply  to  the  west,  while  the  gneiss 
has  gentle  eastward  dips.  This  stratigraphical  break  proves 
the  existence  of  a  fault  on  the  west  side  of  the  synclinal. 
The  down-throw  was  to  the  east;  and  the  high  range  of  hills, 
of  which  Wachusett  forms  the  culminating  point,  is  prob- 
ably a  remnant  of  the  grand  escarpment  produced  by  this 
dislocation  of  the  strata,  and  marks  the  point  where  the 
down-throw  was  greatest.  This  fault,  it  is  believed,  extends 
nearly  or  quite  across  the  State.  The  common  bound- 
ary of  the  first  band  of  mica  slate  and  the  narrow  band  of 
granite  and  gneiss,  marks  the  position  of  the  eastern  of  the 
two  faults  mentioned.  Here,  also,  the  down-throw  was  to  the 
east,  and  has  been  sufficient  to  cut  off  very  obliquely  4,000  or 
5,000  feet  of  mica  slate,  and  bring  the  underlying  gneiss  up 
to  the  present  plane  of  denudation.  The  occurrence  of  ar- 
gillite in  Harvard  and  Bolton,  east  of  the  granite,  proves  that 
the  lateral  displacement  produced  by  the  fault  has  not  been 
less  than  a  mile.  This  fault  probably  extends  as  far  south  as 


36 


Worcester.  It  accounts  perfectly  for  the  present  relative 
positions  of  the  granite,  mica  slate,  and  conglomerate  in  this 
part  of  the  valley  ;  and  we  are  able  to  find  in  the  great  dis- 
turbance, of  which  this  fracture  is  an  index,  a  cause  for  the 
metamorphism  of  the  conglomerate  and  the  conversion  of 
much  of  the  gneiss  along  the  line  of  fracture  into  granite.1 

Beyond  the  northern  boundary  of  Bolton,  the  second  or 
middle  band  of  mica  slate  disappears,  and  reappears  five 
miles  farther  north  near  the  common  boundary  of  Harvard 
and  Ayer.  Here,  also,  the  granite  widens  rapidly,  and  is 
brought,  by  the  disappearance  of  the  mica  slate,  in  contact 
with  the  argillite.  In  the  paper  referred  on  p.  5,  I  have 
shown  that  this  disposition  of  the  rocks  necessitates,  for  its 
explanation,  a  second  fault,  which  begins  in  the  northern  part 
of  Bolton,  and  extends,  nearly  parallel  with  the  fault  just 
described,  along  the  western  border  of  the  granite.  The 
down-throw  of  this  fault  is  on  the  west,  increasing  toward  the 
north ;  and  only  after  the  fault  enters  Harvard  does  the 
down-throw  become  sufficient  to  throw  all  the  mica  slate 
below  the  present  surface. 

.  Between  Ayer  and  the  Merrimac,  there  is,  as  Mr.  Burbank 
has  shown,  a  synclinal  of  mica  slate ;  and  the  mica  slate  is 
conformably  underlaid  on  either  side  by  gneiss.  This  Merri- 
mac synclinal  is  really  a  branch  of  the  main  Nashua  synclinal^ 
but  contains  no  argillite ;  and  it  embraces  a  broad  belt  of 
granite  and  granitoid  gneiss,  which  is  an  expansion  of  the 
narrow  band  extending  southward  through  Harvard  and 
Bolton.  To  raise  this  immense  mass  of  the  underlying 
gneisses  to  their  present  position,  and  leave  the  bands  of 
mica  slate  on  either  side  unremoved  by  denudation,  requires 
•two  faults;  one  on  each  side  of  the  valley,  between  the 

1  Since  writing  this  sketch  of  the  stratigraphy  of  the  Nashua  valley,  I  have 
learned  that,  concerning  the  structure  of  the  region  about  the  Harvard  conglom- 
erate, my  views  bear  some  resemblance  to  those  previously  advanced  by  Mr.  Bur- 
bank.  He,  however,  regards  the  complication  of  this  region  as  due  to  ill-defined, 
vertical  movements  of  partially  plastic  materials,  rather  than  to  well  marked 
faults  in  rockg  essentially  rigid.  Wherever  the  two  views  coincide  his  is  entitled 
to  priority. 


37 


granite  and  the  mica  slate,  with  the  downthrow  in  each  case 
on  the  side  toward  the  mica  slate.  But,  in  Harvard,  we  al- 
ready have  two  such  faults  bounding  tie  granite;  and  we 
have  only  to  conceive  these  to  extend  north-easterly  along 
the  margins  of  the  Merrimac  synclinal,  and  the  structure  of 
the  whole  region  becomes  clear  immediately. 

The  broad,  unbroken  expanse  of  gneiss  between  Wachu- 
sett  and  the  Connecticut  valley  exhibits  several  prominent 
anticlinals  —  prominent  geologically,  but  without  definite 
topographical  representation;  the  corresponding  synclinals 
appear,  however,  to  be  shallow,  as  they  contain  no  mica  slate 
or  argillite.  In  the  Connecticut  valley  the  crystallines  are 
pretty  effectually  concealed  by  the  Triassic  beds;  but  there 
is,  according  to  Prof.  Hitchcock,  much  mica  slate  along  the 
eastern  side  of  the  valley,  in  Amhcrst,  Wilbraham,  and  other 
towns,  which,  for  the  most  part,  dips  to  the  west,  and  appears 
to  overlie  conformably  the  gneisses  to  the  eastward.  The 
well  known  argillite  in  Bernardston  covers  an  area  three  to 
four  miles  broad,  and  has  very  steep  easterly  dips.  On  the 
south,  it  appears  to  pass  under  the  Mesozoic  sandstone,  which 
is  unbroken  in  that  direction  and,  south  of  Northampton, 
fills  the  entire  valley,  making  observations  of  the  underlying 
rocks  impossible.  A  small  patch  of  argillite,  however,  appears 
in  Whately  immediately  west  of  the  sandstone,  and  north  of 
the  granite ;  and,  since  the  argillite  in  Bernardston  extends 
northward  along  the  Connecticut  for  many  miles,  we  may,  I 
think,  safely  conclude  that  it  extends  nearly  or  quite  across 
the  State  in  a  southerly  direction.  West  of  the  argillite  and 
the  overlying  sandstone  is  a  broad  area  of  mica  slate,  which 
stretches,  in  a  N.-S.  direction,  quite  across  the  State,  and, 
like  the  argillite,  extends  a  long  distance  beyond  the  State 
limits.  In  the  eastern  half  of  this  belt  of  mica  slate,  the 
dips  are  very  steep  to  the  east.  Thus  we  find  the  valley  of 
the  Connecticut  underlaid  by  a  grand  synclinal  of  Mont 
Alban  rocks.  This  synclinal  embraces  the  same  rocks  as  the 
Nashua  synclinal,  and  they  are  arranged  in  the  same  order. 


38 


The  small  area  of  gneiss  which  crosses  the  Deerfield  River 
at  Shelburne  Falls,  and  exhibits  a  distinct  anticlinal  structure^ 
bears  the  same  relation  to  the  Connecticut  synclinal  that  the 
broad  gneissic  area  of  which  Wachusett  is  the  highest  point 
does  to  the  Nashua  synclinal.  The  anticlinal  structure  of 
the  Shelburne  gneiss  and  the  overlying  mica  slate,  is  shown 
in  the  section  from  Greenfield  to  Williamstown,  which  ap- 
pears in  the  Report  of  Prof.  Hitchcock  on  the  Geology  of 
Vermont.  A  glance  at  the  map  will  show  that  the  granitic 
area  in  Williamsburg  and  Westhampton  lies  on  the  line  of 
strike  of  the  Shelburne  gneiss,  and  is  elongated  in  a  north 
and  south  direction.  It  represents,  as  I  conceive,. the  exten- 
sion southward  of  the  Shelburne  anticlinal.  And  the  great 
disturbance  displayed  by  the  gneiss  in  the  river  at  Shelburne 
Falls  has  only  been  carried  a  step  further  to  produce  the  ex- 
travasated  granite  of  William sb'urg.  West  of  the  axis  of  this 
anticlinal,  the  mica  slate  exhibits  steep  westerly  dips,  which 
continue  to  the  band  marked  on  the  map  as  talcoid  slate. 
This  extends  nearly  across  the  State,  and  has  very  steep, 
usually  vertical,  dips.1  Beyond  this  band,  to  the  west,  the 
mica  slate  and  the  underlying  gneiss  dip  steeply  to  the  east. 
These  facts  prove  the  existence  along  this  line  of  a  syncli- 
nal similar  to  the  others,  but  in  which  the  argillite  is  re- 
placed by  the  talcoid  slate.  The  probable  unconformability 
of  the  mica  slate  and  talcoid  slate  is  shown  by  the  fact  that 
the  latter,  along  its  western  boundary,  in  the  southern  half 
of  its  extent,  is  underlaid,  not  by  mica  slate,  but  by  gneiss. 
In  the  gneiss  of  the  Hoosac  Mountains,  we  have  another 
great  anticlinal,  which,  in  the  northern  half  of  the  State, 
is  flanked  on  the  east  by  mica  slate,  but  bears  upon  its 
western  slopes  the  semi-crystalline  rocks  of  Berkshire  county, 
already  referred  to. 


il  am  indebted  to  Prof.  B.  K.  Emerson  of  Amherst  College  for  data  relative  to 
the  boundaries  of  the  talcoid  slate  in  Blaiidford,  and  also,  for  information  concern- 
ing the  occurrence  of  emery  in  situ  three  miles  south  of  Chester  on  the  line  of 
strike  of  the  emery  deposits  at  that  place. 


39 


This  brief  and  imperfect  sketch  of  the  stratigraphy  of 
the  region  between  the  western  boundary  of  the  Huronian 
formation  and  the  Hoosac  Mountains,  makes  it  probable 
that  the  strata  are  essentially  continuous  and  conformable 
throughout,  and  belong  to  one  and  the  same  series.  And 
this  view  is  confirmed  when  lithological  comparisons  are 
instituted  between  the  different  portions  of  this  great  area. 
No  important  differences  of  a  general  nature  can  be  shown  to 
exist  between  the  gneisses  of  the  various  regions  named. 
The  gneiss  west  of  the  Connecticut  closely  resembles  much 
that  occurs  east  of  Worcester;  and  it  is  believed  the  same 
may  be  said  of  the  argillites  and  mica  slates.  The  gneiss  and 
mica  slate  east  of  the  Connecticut  are  usually  regarded  as 
essentially  identical  with  the  rocks  of  the  White  Mountains 
— as  typical  Mont  Alban;  and  it  seems  necessary,  in  the 
light  of  the  facts  now  known,  to  conclude,  provisionally  at 
least,  that  the  rocks  of  Wachusett  and  the  Hoosac  range  are 
synchronous.  And,  since  the  rocks  west  of  the  Connecticut 
lie  in  N.-S.  bands,  continuous  with  similar  bands  in  Vermont, 
the  inference  seems  probable  that  the  gneiss  of  the  Green 
Mountains  is  also  of  Mont  Alban  age. 

The  limestone,  quartzite,  talcoid  slate,  and  clay  slate,  in 
Berkshire  county,  appear  on  the  Centennial  Map  substan- 
tially as  on  previous  maps.  In  the  region  of  Great  Barring- 
ton  and  Stockbridge,  however,  some  important  alterations  of 
boundaries  will  be  observed,  which  are  based  principally 
upon  the  published  observations  of  Prof.  J.  D.  Dana.  Prof. 
Dana's  views  concerning  the  order  of  succession  of  these 
various  rocks  and  their  lithological  relations  are,  in  the  main, 
adopted  here.  All  the  rocks  named  seem  to  belong  to  one 
and  the  same  series,  which  is  essentially  conformable  through- 
out. The  limestone  lies  principally  at  the  base  of  the  series, 
and  is  overlaid  on  the  east  by  the  quartzite,  which  is  fre- 
quently in terst ratified  with  and  passes  into  mica  slate  and 
gneiss.  Toward  the  west,  the  quartzite  is  mainly  replaced 
by  mica  slate ;  and  this,  farther  west,  gradually  becomes  the 


40 


so-called  talcoid  slate,  which  is  essentially  hydro-micaceous. 
Thus  the  quartzite,  mica  slate,  and  talcoid  slate  appear  to  be 
stratigraphically  the  same.  And  above  these,  it  is  believed, 
belongs  the  clay  slate,  of  which  only  a  small  portion  lies 
within  the  limits  of  Massachusetts. 

The  age  of  these  rocks  is  regarded  as  yet  an  open  ques- 
tion. This  series  is,  however,  undoubtedly  newer  than  the 
gneiss  of  the  Hoosac  Mountains ;  and  it  seems  probable  that 
it  will  ultimately  prove  to  be,  as  an  increasing  number  of 
geologists  are  inclined  even  now  to  regard  it,  older  than  the 
Primordial ;  i.  e^  may  be  taken,  as  suggested  farther  back,  to 
represent  the  close  of  Eozoic  time.  But,  in  view  of  the 
great  uncertainty  in  which  this  question  is  shrouded,  I  have 
on  the  map  applied  to  the  different  members  of  what  may 
be*  called  the  Berkshire  county  series  no  terms  having  chron- 
ological significance.  The  names  employed  are,  Eolian  lime- 
stone, quartzite,  Taconic  slate,  and  clay  slate.  With  the 
quartzite  are  included  the  gneiss  and  mica  slate  with  which 
it  is  associated.  Taconic  slate  has  no  reference  to  the  form- 
ation so  named  by  Emmons,  but  means  simply  the  slate  of 
the  Taconic  Mountains;  the  object  in  using  this  term  being 
to  distinguish  this  talcoid  slate  from  the  talcoid  slate  farther 
east.  Though  having,  as  already  stated,  no  settled  opinion 
relative  to  the  age  of  these  rocks,  I  have,  however,  in  the  ex- 
planatory column  connected  with  the  map,  grouped  together^ 
partly  for  the  sake  of  convenience,  the  different  members  ot 
.this  series,  and  the  talcoid  slates  to  the  eastward,  and  placed 
the  whole  at  the  top  of  the  Eozoic. 

PALEOZOIC. 

Although  the  fossils  characteristic  of  the  Acadian  group 
have  been  found  at  only  one  locality  in  Massachusetts,  viz.? 
Hay  ward's  quarry,  in  Brain  tree,  yet  most  observers  agree  that 
the  greater  portion  of  the  slates  in  the  vicinity  of  Boston 
are  probably  of  Primordial  age ;  and  I  have  so  represented 
them  on  the  map.  Argillite  occurs  on  Kent's  Island,  in 


BK  . 

OF 


. 

Newbury,  apparently  folded  up  in  the  felsite,  though  possibly 
thrown  down  by  a  fault,  which  has  been  frequently  referred 
to  the  Primordial ;  and  I  have  here  taken  the  same  view  of 
the  matter.  This  argillite  is  associated  with  conglomerate 
which  resembles  some  that  I  have  referred  to  the  Primordial 
in  the  vicinity  of  Boston.  The  hard  metamorphic  slate  in 
the  vicinity  of  Newport,  R.  I.,  has  also  been  referred  provis- 
ionally to  this  horizon. 

The  Norfolk  county  belt  of  conglomerate,  which  stretches, 
with  varying  width,  through  the  towns  of  Braintree,  Ran- 
dolph, Canton,  Norwood,  Walpole,  and  Wrentham,  into 
Rhode  Island,  and  which  may  be  traced  thence,  more  or  less 
continuously,  to  Pawtucket  and  Providence,  has  quite  dis- 
tinctive characters.  It  has  been  well  described  by  Prof. 
Edward  Hitchcock,  and,  later,  by  Mr.  W.  W.  Dodge.  This 
series  is  probably,  as  Prof.  Hitchcock  long  ago  suggested, 
older  than  the  Carboniferous ;  and  yet,  on  the  other  hand,  it 
seems  to  be  newer  than  the  Primordial ;  and  I  have,  therefore, 
followed  Prof.  Hitchcock,  and  referred  it  provisionally  to  the 
Devonian,  assuming  it  to  be  equivalent  to  the  Catskill  or  Old 
Red  Sandstone. 

Gray  conglomerates  and  sandstones,  precisely  similar  to 
the  genuine  Carboniferous,  have  been  observed  along  the 
middle  of  this  Devonian  belt,  in  Wrentham ;  also  by  Mr.  F. 
W.  Very,  in  Walpole,  and  Mr.  W.  W.  Dodge,  in  Canton. 
These  observations  seem  to  indicate  the  existence  of  a 
tongue  of  Carboniferous  rocks  extending  north-east  from  the 
Carboniferous  basin  and  splitting  the  Devonian  band  ;  and  I 
have  ventured  to  represent  it  so  on  the  map. 

The  conglomerate  so  well  developed  about  Boston,  and 
widely  known  as  the  Roxbury  conglomerate,  is  lithologically 
identical  with  the  Carboniferous  conglomerate  of  Bristol 
county  and  Rhode  Island,  and  seems  to  be  similarly  related 
to  the  Primordial  rocks.  It  has  been  frequently  referred  to 
the  Carboniferous  horizon;  and,  in  view  of  the  facts  just 
cited,  and  the  absence  of  any  positive  evidence  to  the  con- 
6 


trary,  this  is  certainly  the  most  probable  view  of  its  age. 
Nevertheless,  it  closely  resembles  the  Primordial  conglom- 
erate of  this  vicinity ;  and,  in  the  absence  of  paleontological 
characters,  we  are  obliged  to  rely  upon  their  stratigraphical 
relations  as  a  means  of  determining  their  relative  distribu- 
tion, and  this,  on  account  of  the  drift-covered  nature  of  the 
country,  seems  wholly  inadequate.  Yet  it  is  probably  safe 
to  regard  as  newer  than  the  Primordial,  the  great  unbroken 
mass  of  conglomerate  lying  between  the  Boston  and  Albany 
Railroad  and  a  line  running  S.  80°  W.  from  Savin  Hill  to 
the  Charles  River. 

Near  the  low  water  line,  011  the  north-east  shore  of  Marble- 
head  Neck,  unconformably  overlying  the '  banded  felsites,  is  a 
hard,  whitish,  fine  grained  sandstone,  or  arenaceous  slate.  I 
have  shown,  in  the  paper  referred  to  on  page  5,  that  it  is 
probably  newer  than  the  Primordial,  and  have  referred  it  to 
the  Carboniferous. 

The  boundaries  of  the  Triassic  formation  in  the  Connect- 
icut valley  have  not  been  altered ;  but  three  shades  of  color 
have  been  used  to  represent  the  three  divisions  of  this  form- 
ation. 

The  eastern  boundary  of  the  Miocene  on  Martha's  Vine- 
yard has  been  altered  in  accordance  with  data  furnished  by 
Prof.  N.  S.  Shaler. 

The  deposit  marked  as  Champlain,  on  the  eastern  end  of 
Nantucket,  has  been  proved  to  belong  to  this  recent  period 
by  the  investigations  of  Mr.  S.  H.  Scudder  and  Mr.  A.  E. 
Verrill.  To  the  same  age  are  referred  the  fossiliferous  de- 
posits in  Truro,  on  Cape  Cod,  and  on  Point  Shirley,  near 
Boston. 


GEOLOGY  OF  THE  NASHUA  VALLEY. 

BY  L.  S.  BURBANK. 


The  Wachusett  range  of  highlands  forms  the  western 
boundary  of  the  valley  of  the  Nashua.  There  is  also,  east  of 
the  valley,  a  parallel  range  of  less  elevation,  extending  from 
Worcester  and  Shrewsbury  to  the  northern  boundary  of  the 
State  in  Tyngsborough.  This  latter  range  is  a  well  marked 
feature  in  the  topography  of  the  region,  and  forms  a  water- 
shed and  climatic  division  of  considerable  importance. 

A  branch  of  the  main  valley  extends  from  the  north  part 
of  Harvard,  north-easterly,  to  the  Merrimac  River.  Two 
other  minor  valleys  extend  westward ;  one  along  the  North 
branch  of  the  Nashua  to  Fitchburg,  another  from  Groton, 
through  Shirley  and  Townsend  to  Ashby. 

The  widest  part  of  the  valley  is  near  the  junction  of  the 
north  and  south  branches  of  the  river  at  Lancaster.  Here 
are  the  broad  and  fertile  intervales  or  flood-plains,  that  at- 
tracted the  attention  of  the  colonists,  and  led  to  the  early 
settlement  of  the  valley.  Near  the  northern  boundary  of  the 
State,  the  hills  on  either  side,  in  Pepperell  and  Dunstable, 
approach  nearer  the  river,  which  here  has  cut  its  channel 
through  sandy  and  barren  plains  of  the  Terrace  Formation 
to  its  junction  with  the  Merrimac.  In  this  sketch  of  the 
geology  of  the  valley,  I  include,  also,  some  observations  on 

(43) 


44 


the  region  bordering  it  on  the  east ;  embracing  the  first  range 
of  highlands  and  a  part  of  the  valley  of  the  Merrimac. 

The  rock  formations  of  this  region  may  be  classified  litho- 
logically,  as  follows : 

1.  Argillite. 

2.  Mica  Slate  and  Quartzite. 

3.  Granite  and  granitoid  Gneiss. 

There  are  also  beds  of  conglomerate  of  small  extent  associ- 
ated with  the  argillite  and  mica  slate;  and,  enclosed  in  strata 
of  gneiss,  are  several  nodular  masses  of  crystalline  magnesian 
limestone,  that  have  been  regarded  by  some  geologists  as 
parts  of  true  beds  contemporaneous  with  the  gneiss. 

The  principal  argillite  beds  form  a  band  extending  north 
and  north-easterly  from  Worcester  to  the  northern  boundary 
of  the  State.  The  widest  part  of  the  band  is  seen  in  the 
towns  of  Lancaster  and  Shirley,  and  here  some  of  the  beds 
are  quite  regular  and  the  lamination  smooth  and  even,  form- 
ing a  good  roofing  slate.  For  the  most  part,  however,  the 
strata  are  much  disturbed  and  contorted,  and  abound  in  veins 
and  lenticular  masses  of  quartz.  The  strike  of  the  strata 
varies  from  nearly  due  north  to  north-east,  averaging,  where 
most  regular,  N.  15°  E.  A  section  across  the  widest  part  of 
the  band  shows  a  uniform  westerly  dip  of  45°  to  90°,  and 
the  mica  slate  on  the  western  border  appears  to  overlie  it 
conformably,  as  if  forming  a  part  of  the  same  series.  Some 
small  areas  on  the  eastern  side  near  the  granite  have  an  east- 
erly dip  at  a  small  angle,  as  in  Harvard,  on  the  west  side  of 
Prospect  Hill.  The  mica  slate  associated  with  the  argillite 
extends  in  a  narrow  band,  southward,  from  Worcester  to 
the  southern  boundary  of  the  State,  where  it  nearly  dis- 
appears, passing  into  gneiss.  In  its  northern  and  eastern  ex- 
tension it  is  separated  by  the  granite,  and  forms  two  distinct 
ranges;  1st,  that  which  encloses  the  argillite  and  lies  mostly 
in  the  valley  of  the  Nashua,  2d,  a  range  which,  bending 
eastward,  extends  in  a  narrow  band  along  the  valley  of  the 
Merrimac. 


Parallel  to  this  latter  range,  on  the  north-west  side,  and  sep- 
arated from  it  by  the  granite,  the  mica  slate  extends  from  the 
Nashua  River  in  Groton  to  the  Menimae  in  Tyngsborough, 
in  a  band  having  a  uniform  southeasterly  dip,  thus  forming  a 
synclinal  which  includes  the  granite.  Beyond  the  Merrimac  in 
this  direction,  the  slate  disappears,  passing  into  gneiss.  That 
this  narrow  range  of  slate  belongs  to  the  same  system  with 
the  Lowell  slates  is  indicated,  not  only  by  its  position,  but 
by  similarity  in  mineral ogical  character.  A  peculiar,  choc- 
olate-colored, quartzose  mica  slate,  that  abounds  in  the 
vicinity  of  Pawtucket  Falls,  in  Lowell,  is  exactly  identical, 
in  appearance,  with  slates  from  the  steatite  quarry  in  West 
Groton,  and  from  several  other  localities  in  Groton,  Duns- 
table,  and  TyngsboroKgh. 

The  slates  on  the  eastern  side  of  the  granite  range  are  very 
variable  in  mineralogical  character,  but  all  become  coarser 
toward  the  south-eastern  border,  and  finally  pass  into  gneiss. 
In  the  vicinity  of  Lowell,  there  are  numerous  beds  of  very 
coarse  feldspathic  gneiss,  interstratified  with  the  mica  slate. 
The  same  structure  is  seen,  also,  throughout  the  range,  as  far 
south  as  Bolton.  Further  south,  in  Clinton  and  Boylston, 
numerous  bands  of  quartzite  are  interstratified  with  the  slate, 
which  here  crops  out  abundantly,  forming  some  of  the  high- 
est hills  of  the  range  east  of  the  Nashua  valley. 

In  Harvard  and  Bolton,  east  of  the  granite  range  and 
closely  connected  with  the  slates  just  described,  beds  of  con- 
glomerate occur,  interstratified  with  argillite,  which  here 
coincides  in  strike  and  dip  with  the  mica  slates.  (Strike,  N. 
65°  to  70°  E.  Dip,  45°  to  80°  N.  W.)  This  conglomerate 
is  very  interesting  from  its  relations  to  the  surrounding 
rocks,  and  the  remarkable  changes  that  portions  of  it  have 
undergone.  It  appears  in  a  range  extending  from  Pin  Hill, 
in  Harvard,  to  the  summit  of  Wattoquotoc  in  the  south-west 
part  of  Bolton,  forming  several  high  hills,  that  are  marked 
features  of  the  landscape.  The  series  of  specimens  that  I 
have  placed  in  the  Cabinet  of  the  Society  shows  that  the 


pebbles  have,  in  many  cases,  been  flattened,  bent,  and  even 
drawn  out  into  layers,  giving  an  agate-like  structure  to  some 
of  the  rock.  While  the  specimens  collected  indicate  some  of 
the  changes,  only  a  careful  study  of  the  localities  can  show 
how  complete  is  the  transition  from  a  sedimentary  to  a  crys- 
talline rock.  The  principal  conglomerate  beds  lie  between 
hills  of  poryhyiitic  granite  on  the  west  and  north,  and  mica 
slate  and  gneiss  on  the  south-east,  yet  not  a  pebble  of  granite 
or  gneiss,  so  far  as  I  have  seen,  enters  into  its  composition, 
and  the  slaty  pebbles  that  occur,  appear  to  belong  to  argillite 
rather  than  mica  slate.  Some  ten  miles  to  the  south-west, 
however,  are  extensive  beds  of  slate,  interstratified  with  a 
quartz! te  closely  resembling  the  pebbles  that  form  the  mass 
of  the  conglomerate.  The  conglomerate  beds  are  most  abun- 
dant and  least  altered  in  Harvard,  at  the  most  northern 
locality  where  they  appear.  On  tracing  them  southward, 
in  the  direction  of  the  strike  of  the  strata,  they  are  seen  to 
become  more  altered  and  less  distinct  in  character,  until  they 
are  completely  blended  with  enclosing,  conformable  strata 
of  mica  slate.  In  a  large  exposure  of  the  rock,  on  the  sum- 
mit of  Wattoquotoc,  in  Bolton,  only  a  few  small  spots  can  be 
found  where  the  conglomerate  structure  can  be  made  out. 
Further  south  the  rock  is  entirely  crystalline,  showing  no 
traces  of  mechanical  origin. 

The  rock  which  immediately  encloses  the  beds  of  slate  and 
conglomerate  at  Harvard  is  of  a  very  peculiar  character,  as 
shown  by  specimens  that  I  have  placed  in  the  Society's  Col- 
lection. It  appears  in  unstratified  masses,  resembling  granite, 
but  mixed  with  fragments  of  slate,  and  destitute  of  mica; 
and  also  as  a  schistose  feldspathic  gneiss  in  which  slaty  part- 
icles take  the  place  of  mica,  while  the  feldspar  is  in  imperfect 
rounded  crystals,  giving  a  porphyritic  aspect  to  the  rock. 

The  area  colored  to  represent  granite  and  granitoid  gneiss 
is  much  larger  than  has  hitherto  appeared  on  any  geological 
map  of  the  region.  It  covers,  however,  only  the  region 
where  granite  predominates,  though  there  are,  especially  in 


the  north-eastern  division,  near  the  Merrimac  River,  some 
intercalated,  beds  of  well  stratified  gneiss.  From  Groton 
southward  to  Boylston,  this  granite  is  decidedly  porphyritic,. 
and  in  Clinton  and  Berlin,  where  it  is  largely  developed,  the 
crystals  of  orthoclase  are  very  large  and  perfect,  often  two 
inches  or  more  in  diameter.  Further  south  it  passes  into 
porphyitic  and  ordinary  gneiss.  I  have  included  .the  granite 
of  Worcester  in  the  same  formation,  though  it  cannot  be 
shown  that  it  is  actually  connected  with  the  same  range. 
It  may,  perhaps,  be  more  properly  classed  with  some  smaller 
out-crops  of  granite  that  occur  among  the  slates  west  of  the 
principal  range. 

From  the  facts  observed,  it  seems  reasonable  to  infer  that 
this  granite  is  a  metamorphic  rock,  produced  by  alteration  of 
deeply  buried  sediments  underlying  the  slates,  together  with 
some  portions  of  the  same  series  of  slates  and  conglomerates 
that  appear  in  their  partly  altered  condition  at  the  surface. 
The  least  altered  of  the  slates  and  conglomerates  probably 
formed  the  highest  parts  of  an  elevated  range,  which  settled 
to  its  present  position  simultaneously  with  the  elevation  of 
the  softened  material  which  formed  the  granite  -range. 

The  gneiss  on  the  south-eastern  side  of  the  range  of  slates 
contains  the  magnesian  limestones  to  which  I  have  referred. 
Several  of  the  larger  deposits  occur  in  a  line  coinciding  with 
the  strike  of  the  gneiss,  which  extends  through  Bolton,  Box- 
boro',  Littleton,  and  Chelmsford.  I  have  elsewhere  1  given 
reasons  for  holding  that  these  limestones  are  not  true  beds, 
but  segregated  masses  of  a  vein-like  character,  derived  by 
solution  from  the'  enclosing  strata  of  gneiss.  This  view, 
which  was  maintained  in  opposition  to  the  theory  of  their 
organic  origin,  was  not  altogether  new,  as  may  be  seen,  by 
the  following  quotation  from  Hitchcock's  Geology  of  Mass. 
(1841),  p.  563. 

"  The  geological  situation  of  these  limestone  masses,  and  their 
mineral  character,  are  so  similar  that  one  description  will  embrace 

1  See  Froc.  Boston  Soc.  of  Nat.  History,  Vol.'  xiv,  p.  19f>.      '     • 


48 


them  all.  They  all  correspond  to  the  description  which  Dr.  Maccul- 
loch  gives  of  the  limestone  of  Tirey,  one  of  the  Western  Islands  of 
Scotland  ;  'It  is,'  says  he,  '  improperly  called  a  bed,  as  it  is  only  an 
irregular  rock,  lying  among  the  gneiss,  without  stratification  or  con- 
tinuity. In  this  respect,  it  resembles  the  greater  number  of  primary 
limestones  found  in  gneiss  and  mica  slate,  and  may  be  considered 
as  a  nodule.'  " 

It  is  true  that  the  rock  at  Chelmsford  and  Bolton,  which  con- 
tains the  so-called  Eozoon  Canadense  has  some  appearance  of 
stratification,  but  this  is  for  the  most  part  a  banded  structure, 
which  is  seen  only  on  and  near  the  walls  of  the  cavities, 
while  the  mass  of'  the  limestone  shows  no  appearance  of 
stratification.  It  is  not  improbable,  however,  that  some  por- 
tions of  the  impure  limestone  of  the  walls  may  be  partially 
altered  strata  of  the  gneiss.  It  is  a  fact  of  some  significance 
also,  that,  while  the  unstratified  mass  of  the  deposits  is  dolo- 
mite, the  limestone  of  the  walls  is  nearly  pure  calcite,  filled  and 
penetrated  in  every  direction  with  crystals  of  various  mag- 
nesian  silicates,  and  the  serpentine  takes  various  concretion- 
ary forms,  or  appears  as  chrysotile  veins  traversing  the  ser- 
pentine masses  and  granules,  and  sometimes  investing  them, 
forming  a  fibrous  wall  on  their  surfaces,  as  they  occur  im- 
bedded in  the  calcite.  In  these  localities,  the  eozoonal  forms 
occur  only  near  the  walls  of  the  cavities  and  associated  with 
other  minerals;  never  in  the  principal  masses  of  the  lime- 
stone. At  Chelmsford,  the  gneiss  enclosing  the  limestone 
has  clearly  an  anticlinal  position;  at  the  other  localities,  this 
structure  is  not  so  plainly  shown,  but  I  think  the  same  theory 
will  account  for  all  the  deposits,  viz.,  that  they  are  vein-like, 
concretionary  masses,  filling  cavities  in  the  gneiss,  and  bccur- 
ing  especially  along  the  line  of  a  nearly  closed  and  reversed 
anticlinal  fold.  The  great  number  of  mineral  species  associ- 
ated with  this  limestone,  some  of  which  are  characteristic 
vein  minerals,  tends  to  confirm  this  view.  The  following  is 
a  list  of  minerals  found  in  the  limestone  of  Bolton,  Box- 
boro',  Littleton,  and  Chelmsford :  Scapolite,  boltonite,  nut- 
talite,  chondrodite,  petalite,  sphene,  pyroxene,  diopside,  apa- 


49 


tite,  actinolite,  asbestus,  augite,  allanite,  coccolite,  pargasite, 
chromite,  pyrite,  magnesite,  phlogopite,  talc,  ophite,  chrys- 
otile,  satin  spar,  rhomb  spar,  allanite,  spinel,  garnet. 

In  the  vicinity  of  Lowell,  the  slates  are  intersected  by 
numerous  dykes  of  diorite,  which  sometimes  occur  as  appar- 
ent beds  included  between  the  layers  of  mica  and  hornblende 
slate.  One  of  these  dykes  at  Pawtucket  Falls  is  interesting 
as  showing  fine  examples  of  boulders  of  disintegration.  A 
dyke  which  is  included  between  nearly  perpendicular  strata 
of  slate  appears  at  the  surface  as  if  composed  of  numerous 
boulders,  closely  packed,  and  somewhat  regularly  arranged. 
Some  of  the  rounded  masses  have  become  detached,  and 
slightly  water-worn,  and  these  cannot  be  distinguished  by 
their  appearance  from  boulders  of  trap,  in  the  ordinary  drift. 
Whether  an  original  concretionary  structure  in  the  rock  has 
determined  the  size  and  form  of  these  boulder-like  masses ; 
or  whether  ordinary  decomposition  and  exfoliation,  beginning 
on  the  corners  and  edges  of  blocks  resulting  from  a  jointed 
structure,  has  produced  them,  may  be  doubtful.  In  either 
case,  the  facts  indicate  that  some  of  the  rounded  boulders  of 
the  drift  may  owe  their  forms  less  to  any  mechanical  action 
of  wearing  and  grinding,  than  to  such  a  process  of  previous 
decomposition  and  exfoliation.1 

A  small  trap  dyke,  three  or  four  inches  in  width,  that  tra- 
verses the  gneiss  and  limestone  in  one  of  the  quarries  at 
Chelmsford,  exhibits  an  irregular  lamination,  causing  it  to 
separate,  when  broken,  into  transverse,  flattened,  somewhat 
prismatic  layers.  As  these  occur  in  place  in  the  dyke,  they 
are  uniformly  bent  upward  in  the  centre,  showing  that  the 
material  when  forced  into  the  fissure  was  in  a  plastic  condi- 
tion and  adhered  to  the  walls  on  each  side.  The  trap  rock 
of  the  numerous  dykes  near  the  Pawtucket  Falls,  in  Lowell, 
abounds  in  joints  which  divide  it  into  small  blocks.  These 
blocks  are  often  coated  with  a  black,  or  dark  green  mineral, 
generally  striated  or  semi-fibrous  in  structure,  and  having  a 

1  See  Proc.  Boston  Soc.  of  Nat.  History,  Vol.  XVI,  p.  150. 


50 

brilliant,  polished  appearance  on  the  surface,  exactly  resem- 
bling, in  some  specimens,  the  appearance  on  the  walls  of 
veins,  which  has  been  called  slicken-sides.  At  Lowell,  such 
layers  occur  on  all  sides  of  angular  blocks,  in  positions  show- 
ing that  their  polished  appearance  cannot  be  due  to  any 
mechanical  process  of  grinding  or  sliding  over  another  sur- 
face of  rock.  These  coatings  are  generally  very  thin,  but 
sometimes  occur  in  layers  of  half  an  inch  or  more  in  thick- 
ness, forming  a  distinct  mineral,  which  in  physical  and  chem- 
ical characters  closely  resembles  delessite. 

In  regard  to  the  position  in  the  geological  scale  of  the 
rock  formations  here  described,  I  have  no  very  positive  opin- 
ions to  offer.  I  have  been  inclined  to  regard  the  gneiss  east 
of  the  Nashua  valley  as  belonging  to  a  distinct  system  older 
than  the  Wachusett  gneiss.  The  mica  slate,  also,  which  is 
interstratified  with  it  and  overlies  it  conformably,  evidently 
belongs  to  the  same  continuous  series.  On  the  western  bor- 
der of  the  valley  the  mica  slate  has  still  a  westerly  dip,  and 
appears  to  pass  under  the  nearly  horizontal  strata  of  the 
Wachusett  gneiss.  The  peculiar  concretionary  veins  con- 
taining beryl  and  tourmaline  crystals,  which  characterize  the 
Wachusett  gneiss,  do  not  occur  east  of  the  mica  slate.  The 
slates  containing  andalusite  and  staurolite  are  abundant  on 
the  western  border  of  the  valley,  but  do  not  occur  east  of 
the  argillite.  The  gneiss  east  of  the  valley  is  generally  more 
compact,  heavier  and  less  micaceous  than  that  of  the  Wachu- 
sett range. 

The  argillite  beds  also  appear,  for  the  most  part,  to  occur 
in  strata  conformable  with  the  mica  slate,  and,  on  the  west- 
em  border  especially,  they  are  so  blended  that  no  well 
marked  boundary  line  between  them  can  be  laid  down.  The 
andalusite  crystals  characteristic  of  this  part  of  the  forma- 
tion occur  abundantly  in  both  the  mica  slate  and  argillite. 
The  argillite  beds  are  generally  highly  inclined,  and  in  many 
places  nearly  in  a  vertical  position.  They  are  also  much 


51 

contorted,  and  filled  with  veins,  layers,  and  lenticular  masses 
of  quartz. 

For  reasons,  some  of  which  I  have  stated  above,  I  have 
heretofore  regarded  the  gneiss,  mica  slate,  and  argillite,  as 
forming  a  continuous  series,  all  older  than  the  gneiss  of  the 
Wachusett  range.  Mr.  Crosby  has  proposed  a  different  view, 
which  will  explain  most  of  the  facts  quite  satisfactorily,  on 
the  theory  that  the  strata  have  been  brought  into  their  pres- 
ent relations  by  the  occurrence  of  several  faults,  by  which 
the  slates  have  been  brought  in  contact  with  the  granite  and 
gneiss.  While  I  have  no  doubt  that  vertical  movements  to 
a  considerable  extent  have  occurred,  especially  among  the 
rocks  east  of  the  valley  in  Harvard  and  Bolton,  I  think  there 
is  evidence  showing  that  these  movements  took  place  when 
some  of  the  rocks  were  in  a  highly  plastic,  and  almost  fluid 
condition,  so  that  the  porphyritic  granite  of  Harvard,  for 
example,  though  undoubtedly  metamorphic,  may  also  be  re- 
garded as  an  eruptive  mass. 

With  the  exception  of  the  granite  and  gneiss  used  for 
building,  few  valuable  products  are  obtained  from  the  rock 
of  this  region.  The  argillite  of  Lancaster  was  formerly 
quarried  for  roofing  slate,  and  recently  the  debris  from  the 
old  quarries  has  been  ground  into  "  slate  flour"  to  be  used  as 
paint,  and  in  compositions  for  roofing.  The  metamorphic 
slate  of  Harvard  was  formerly  sawed  into  slabs  and  used  for 
gravestones  and  other  purposes,  but  these  quarries  have  not 
been  worked  for  many  years. 

In  Leominster,  a  fine  arenaceous  mica  slate  has  been  man- 
ufactured into  scythe  stones,  which  have  acquired  a  consider- 
able local  reputation,  the  farmers  of  the  neighboring  towns 
generally  preferring  them  to  all  other  kinds. 

The  manufacture  of  lime,  which  was  carried  on  more  than 
fifty  years  ago,  at  Bolton,  Chelmsford,  Boxboro'  and  Littleton, 
was  continued  at  Bolton  till  about  fifteen  years  ago.  The 
other  quarries  were  abandoned  many  years  before.  I  have 
learned  that  the  work  was  abandoned,  in  most  cases,  on  ac- 


52 


count  of  the  exhaustion  of  the  limestone,  or  the  increased 
difficulty  in  quarrying  it,  rather  than  from  any  lack  of  demand 
for  the  lime. 

There  are  numerous  veins  and  nodules,  and  sometimes 
intercalated  beds  of  ferruginous  quartz  associated  with  the 
argillites  and  mica  slates  of  the  Nashua  Valley.  This  quartz 
is  of  a  vitreous  character,  and  entirely  distinct  from  the 
quartzite  layers  that  are  abundant  in  the  older  slates. 

The  close  resemblance  of  these  rocks  to  the  gold-bearing 
quartz  of  North  Carolina  and  Virginia,  is  certainly  very 
striking.  Many  excavations  have  been  made  in  them  in 
search  of  gold.  I  do  not  know  that  any  proper  test  has 
been  made  to  determine  the  question  whether  any  gold  is 
really  to  be  found  in  them.  I  have  examined  many  speci- 
mens from  different  localities,  with  the  microscope,  but  find 
no  gold  visible.  These  veins  are  especially  numerous  near 
the  eastern  border  of  the  argillite  in  Harvard  and  Boylston. 
At  the  latter  place  I  have  examined  several  openings  made 
by  returned  Californian  miners,  where  the  decomposing  sul- 
phides are  very  abundant  in  the  quartz  veins,  and  I  am  not 
surprised  that  the  attention  of  persons  familiar  with  gold- 
bearing  veins  of  other  regions  was  attracted  to  these 
localities. 

New  excavations  have  recently  been  made  in  the  old  lead 
mine  in  the  south  part  of  Sterling,  but  nothing  of  value  was 
obtained.  It  is  not  uncommon  to  find,  in  the  quartz  veins 
and  nodules  enclosed  in  the  slates,  some  traces  of  galena  and 
copper  pyrites,  as  well  as  of  the  ores  of  iron,  but  no  well- 
marked  veins  of  any  value  have  yet  been  discovered  in  any 
part  of  the  region  that  I  have  explored. 

The  anthracite  of  Worcester  was  fully  described  in  Prof. 
Hitchcock's  Report.  No  other  similar  beds  have  been  found 
in  the  formation,  and  this  has  not  been  found  valuable,  nor 
explored  to  any  considerable  extent.  No  fossils  having  been 
found  in  the  associated  slates,  the  deposit  is  not  specially  im- 
portant in  its  geological  significance. 

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